After Jewelius has usurped the princesses, he sentences the dishonored heroes into exile or slavery that same night, having already signed treaties with Diamond Dogs and dragons as well as having a slave galley for Applejack and Pinkie Pie. He may have manipulated all of Canterlot into accepting him as their king, but how is he able to make an antislavery nation to accept these radical changes as well as sign treaties with other nations so quickly? It would be possible if things are as explained below.
He has travelled a lot in order to find a way to take Equestria's throne for himself. After all, it was through these travels that he learned about the Changelings and the location of their kingdom as well as the caverns underneath Canterlot. It could be possible that he secretly made connections with other nations even before allying with Chrysalis in preparation of making it easier for the other nations to accept Equestria's new leadership. The announcer of the dragons' coliseum explicitly states that Jewelius gave it to them as part of their treaty with him. It's never discussed if the details of the treaty were first discussed before or after his takeover.
As for the matter of slavery being accepted just like that, Jewelius never publicly legalizes slavery. It would do no favors to his façade of a well-meaning ruler if he started practicing slavery in Canterlot immediately after his takeover. It's safer to practice slavery from a distance in areas where it's more accepted. Neither Canterlot nor Ponyville's citizens ever talk about the dishonored heroes being in slavery, they only talk about them being banished. In the sentencing scene of the sixth chapter, there's nopony else but the heroes, Jewelius, Vivian and the guards in the throne room to hear the sentences. All the public knows is that the dishonored heroes have been driven away from Equestria as their punishment. They know nothing about slavery being practiced in the locations the heroes are banished to.
Jewelius sentencing the Mane Five and Shining Armor into different forms of slavery is not just because they're deemed to be too dangerous if allowed to remain together. He wants to humiliate each of them by preying on their personal dislikes and robbing them of their individual subjects of pride.
The hardworking farmer Applejack is forced to work in an extremely backbreaking job of a galley slave from which she doesn't get even thanks. The hyperactive Pinkie Pie is denied the joys of having fun or spreading it. They both have to spend most of their time below the deck where there's no fresh air or sunshine, and even their fellow slaves hate them.
The athletic Rainbow Dash has to fight for her life as a gladiator, but she still doesn't get from the crowd the praise she likes to bask in.note She copes by taking pleasure from defying the dragons' expectations of her going down, though. For the timidFluttershy who fears dragons and hates fighting, having to fight for the entertainment of dragons is a nightmare.
Once again Rarity is forced to find and move gems in the dirty mines of the greedy Diamond Dogs, but this time she cannot even retaliate verbally and drive them into releasing her because of the muzzle Jewelius specifically provides to keep her quiet. She's later gifted with her bridesmaid dress (her own piece of work and former subject of pride) as a reminder of her fall from grace. Shining Armor, the Captain of the Royal Guard and groom of the Princess of Love, is reduced to being just a mining slave right after his bride and sister spent some time imprisoned in an abandoned mine.
There are practical reasons for Jewelius' decision to simply banish the princesses from Equestria instead of having them enslaved. They may have lost their authority and prestige in Canterlot, but that doesn't mean they cannot initiate a revolution of their own by gathering supporters elsewhere or finding other authority figures who trust them. Just look at how everypony in Canterlot still loves Cadance and how Celestia extinguishes everypony's hatred of her with humble apologies alone when she's about to be hanged. Besides, it's unlikely that whatever friends the princesses have among foreign authorities would appreciate if they heard they've been reduced to being slaves. That's why it's better to send them to live in isolation (in the prologue, Celestia is in a desert, while Luna is on a snowy mountain). Jewelius admits in the 11th chapter that he hoped that being forced to live a rougher life than they're used to would kill them off. As for Spike, he's too young to be much use to slavers.
Shining Armor saying in the 12th chapter that Jewelius is using Twilight to make himself king may come off erroneous because Jewelius has already proclaimed himself Equestria's king. However, when you consider that Jewelius plans to marry Twilight and have her bear him powerful offspring to continue his legacy, Shining Armor's choice of words starts making sense, for continuing the royal line is often considered an integral part of being a monarch.
This would also serve as a Hoist by His Own Petard case for Jewelius: Chrysalis is able to escape and ultimately kill him because he employed the incompetent guard who makes it possible for the Changelings to escape.
When Jewelius lies to Twilight about her former loved ones' true intentions in the flashback scene of the 14th chapter, he says that what sealed their fates is that they chose to ignore the reality and see the world the way they wanted it to be. Jewelius himself stubbornly refuses to acknowledge the need to prepare against the escaped Changeling army and devotes all his resources in destroying the heroes. He ends up revealing his true nature to Twilight when she insists on disagreeing with him on the matter and says that burying the hatchet with the fugitives would be best for everypony, including himself. She then escapes (with some assistance from a disguised Changeling) to reconcile with her loved ones, using with her friends the Elements of Harmony she took with her to stop Jewelius' army that is attacking Ponyville. Once the heroes have cornered Jewelius, the Changelings make their move and kill Jewelius as revenge for double-crossing them. In short, Jewelius seals his own fate by letting his delusions cloud his judgement.
The same applies to Jewelius' main minions. In the Motive Rant Vivian makes during the final battle, she claims that Jewelius will bring ponies the independence she believes they need to take care of themselves, even though his actions and true motives (of which she's fully aware of) don't support this. Commander Hildread doesn't like the idea of mass murdering all of Ponyville's inhabitants, but she quells her hesitations and stubbornly clings to her opinion that she protects ponies better than Shining Armor or the other heroes with her extremely brutal methods. Only Lieutenant Shackle admits that what they're doing is only evil and helps the heroes to stop the attack. Even though she willingly goes to prison with the rest of Jewelius' conspirators, she vows to be a better and wiser pony after her sentence is served.
In contrast to Jewelius, the heroes (as well as the above-mentioned Shackle) own up to their mistakes and admit some unpleasant truths over the course of the story, and thus ensure their own fates won't be sealed in a bad way. For example in the 12th chapter, when Celestia waits to be hanged, she accepts the unpleasant truth that Twilight and everypony else are too much wrapped around Jewelius' hoof as she silently listens to the other heroes' futile attempts to make everypony believe their Cassandra Truth about Jewelius. When he gives her a chance to make a final statement, she uses it to apologize to Twilight and everypony else for letting them down, saying that she's responsible over everypony who died and suffer now because she placed her trust in the wrong ponies (that is, failing to see Jewelius and the impostor Cadance for what they truly were until it was too late). As she prepares to die without resistance, the crowd and Twilight lose all their animosity toward their former ruler and plead for Jewelius to spare her, watching in horror as she falls through the trapdoor. Even if Luna had failed to save Celestia, Jewelius would have made Celestia a martyr when he wanted her to be forever remembered as the failed monarch he painted her as. By accepting the awful truth that nopony looks up to her anymore and admitting her own share of the responsibility over it, Celestia ensures that even if she dies, her former subjects and faithful student won't remember her with hatred anymore.
Another good example is during the reconciliation scenes between Twilight and her friends, mentor, and brother in Chapter 17. They all point out the screw-ups that they themselves performed instead of anyone else's. This helps everyone accept that they're all responsible for everything that's happened, as they're refusing to blame anybody else. In the case of Twilight and her friends, it also allows them to repair their friendship so they're able to use the Elements of Harmony to stop the attack on Ponyville.
Earlier, Shining Armor tries to explain to the original bridesmaids that Chrysalis fired them on the excuse that they just wanted to meet royalty in the wedding. Highly offended that such lies were believed about her, Twinkleshine starts sobbing, unintentionally putting Shining in the same boat Chrysalis put Twilight into.
Shining Armor: I didn't mean to hurt her feelings! Twilight: But you did! Just like me.
When Jewelius calls Celestia out of failing to realize her niece was being impersonated and allowing the wedding to proceed while the threat of unknown attackers was in the air, she becomes very nervous and impotently asks him to calm himself, all the while Vivian points her horn threateningly at her, and the wedding guests and some guards witness this all. After centuries of being practically worshipped, Celestia isn't used to being told off this openly and aggressively by her subjects or one of her nephews whom she has until now known as a kindly stallion. That — combined with all the nasty surprises that occurred from the moment Twilight and Cadance stopped the wedding ceremony — explains why Celestia isn't clearheaded enough to offer any other explanations than the frantic way Twilight accused Chrysalis without concrete evidence (which does explain why she wasn't taken seriously, but not really anything else concerning the entire mess). As for why she doesn't take a firmer stance against Jewelius, the last time she did so was at the rehearsal when he tried to defend Twilight in a reasonable way. She immediately commanded him to be silent, and given how the events turned out, she feels she's in no position to act like that again. Plus, being on the receiving end of an earful from one of her relatives would likely remind her of her own little sister's Start of Darkness. After she makes in her panic the mistake of telling the heroes to run and is subdued, her guards take her to the dungeons on her nephew's orders while the wedding guests only glare resentfully at her. All of this explains why she has nothing to say in the sentencing scene; she's too shocked by her world falling apart in one day.
Luna isn't shown to be suffering from her usurpation as badly as Celestia, and she tries to reason with Jewelius in the sentencing scene. Unlike her big sister, she has experienced the lack of respect from her subjects' part, both before and after her period as Nightmare Moon. To her, it isn't a hard pill to swallow as much as it is to Celestia because she has experienced it before.
Jewelius has more reason than simple sadism to make an Evil Laugh at the end of the tenth chapter. He was at the rehearsal to hear all the bad words that were said to Twilight as her brother, friends and mentor all left her to check on the "upset" Chrysalis. After he pretended to reluctantly leave with them, he spied on Twilight and witnessed her being sent into the caverns by Chrysalis. In the tenth chapter, he injures Cadance and hides as Twilight, Shining Armor, Celestia, the Mane Five and Spike all find her. He then listens from the shadows as Twilight delivers a bitter "The Reason You Suck" Speech to her former loved ones and leaves them at Commander Hildread's mercy to check on Cadance who's been taken to the medical wing, using the same parting words Shining Armor, Applejack and Celestia used while leaving her at the rehearsal. After she has left, Jewelius appears, and when Shining Armor tearfully answers to his reproaches with "I'm sorry.", Jewelius gives him the same reply Chrysalis gave Twilight when the latter said the same words her brother is now saying: "You will be!" The sheer irony of the entire situation is delightful to him.
Jewelius subverts in more ways than one the Purple Is Powerful motif which Twilight and Shining Armor play completely straight. Purple commonly symbolizes nobility, royalty, wisdom and magic. However, it has also been known to symbolize decadence, conceit, and pomposity. The only purple thing Jewelius has is his royal suit, but unlike with Twilight and her brother, his magic isn't extraordinarily powerful, and he isn't a fighter in any sense of the word. He first introduces himself as a noble and humble prince quite well, but he's proven to be anything but noble and humble. He's poisonous though, influencing Twilight as well as the kingdom of Equestria negatively (though Twilight ultimately chooses justice and forgiveness over self-destructive feuds, playing straight the nobility of the purple color). The detail of having purple only in his clothing nicely paints Jewelius as a poisonous fraud in contrast to the noble-minded and powerful Twilight and Shining Armor who both actually become royals later on.
In the 11th chapter, Fluttershy asks from Jewelius that if he aimed to overthrow the princesses, why he torments her and the rest of the fugitives. He answers: "It was nothing personal really. Most of you were in the wrong place at the wrong time. However, there was 'who would possibly want to do anything that could ruin such a glorious event like the wedding of the wonderful Princess Mi Amore Cadenza'. Let's just say what you all said that time didn't sit too well with me." When you think about it, his claim of it being nothing personal isn't completely untrue, at least not from his sociopathic point of view.note Real-life sociopaths are capable of ruining anyone's (stranger or otherwise) life For the Evulz even when they really have nothing personal against their victims. Because it was Rarity who said that line he hated, he taunts her by sending her bridesmaid dress to her. Aside from sending Spike the wedding cake toppers, publicly flogging Shining Armor, and oppressing the Mane Five's hometown, he doesn't otherwise single out the non-princess exiles in the ways he tortures them.
It may be jarring to some how easily Twilight falls for Jewelius' lies about how her loved ones didn't really care about her after he has banished them and she has woken up from her Power-Strain Blackout. However, at that point, she's not entirely ready to give up on them and tries to convince Jewelius to give them all a second chance. After that, he has a full week of time to work on how to make Twilight think negatively about the exiles while he's still retaining his charming façade around her, culminating in Twilight renouncing her brother, friends and mentor upon their reunion because she mistakenly thinks they caused Cadance to become injured by bringing the latter to Canterlot before Jewelius could reinstate her (as he had falsely promised Twilight). Besides, several details from the time before and after everything that happened at the rehearsal (see below) have given Twilight some understandable doubts in regards to her loved ones. By keeping Twilight's attention in these details as well as undermining her own mistakes by emphasizing her well-meaning motives, Jewelius convinces Twilight that her loved ones were entirely to blame for their falling out.
After Twilight stops the invasion, the first thing Celestia says to Twilight is to assure she herself is alright and that her student has a real wedding to put together (instead of, for example, asking if Twilight — who's exhausted from stopping the Changelings with the shrinking spell — is alright, or apologizing for her cold demeanor toward her). During the outlash Jewelius gives the heroes immediately after this, the wording of the only defence Celestia gives makes it sound like she's trying to pin all the blame on Twilight. Twilight falls unconscious after Celestia tells everypony to run, so she's unaware that Celestia stays behind to look after her and expresses concern for her when she's subdued. Besides, Jewelius can appeal to his status as Celestia's nephew and claim that he knew her better than most ponies to know she wasn't as warm and caring as she presented herself to be.
Shining Armor confesses on Jewelius' pressure that he was acting of his own accord when he banned Twilight from the wedding instead of doing it entirely because of Chrysalis' brainwashing. It breaks Twilight's heart to hear that her BBBFF wasn't acting entirely under the effects of brainwashing, even though he's visibly remorseful. Since she hadn't heard from him since moving to Ponyville before receiving the invitation (which didn't come from him) to his wedding when it was around the corner, and he had no trouble banning her from his wedding because she overreacted in order to protect him, she has every reason to suspect that Jewelius is right and Shining Armor no longer cares about his little sister as much as before.
Jewelius claims that Twilight's friends associated with her only so that they could take advantage of her connections. After all, during the wedding preparations, they brushed off Chrysalis' bridezilla behaviour toward them and praised her, quickly came to the conclusion Twilight's problem with Chrysalis was jealousy, had forgotten the promise they gave her in "Lesson Zero" (as they themselves realize when Twilight brings this up), and quickly tagged along with Shining Armor to check on the "distraught bride", even though they had only just met those two and Twilight was already ashamed of her actions. In light of all of that, it's very tempting to suspect they were kissing up to royalty and abandoned Twilight when they no longer needed her for that. As for the matter of helping Twilight during the invasion and Applejack apologizing to her, Jewelius can argue it was only a matter of survival and saving face.
This thinking is flawed by the question of how the Mane Six could have used the Elements of Harmony if their bonds were false. Twilight can rationalize that their bonds used to be real in the instances they used the Elements (the last time was against Discord), but since then, the others have forgotten to see intrinsic value in their relationship with Twilight. This is alluded to when she announces her decision to reconcile with her loved ones and says she believes they have learned their lesson. Though it took some time to take effect, their insistent attempts to get back on speaking terms with her in spite of her sullenness toward them, combined with her getting bad emotions out of her system, restored her trust in them and made her inclined to give them the benefit of the doubt.
Why Jewelius decides to have Twilight's parents captured for a bargaining chip instead of trying to convince them as well of his good intentions? Because his goal is to mold Twilight into his faithful pawn, and he's using in it a classic tactic of manipulation: isolation from other relationships. It's one thing to charm a lady, but it's another to win her parents' trust, especially when you've recently exiled their other child as well said other child's bride who also used to look after their daughter. If they were allowed to interact with Twilight, they'd likely encourage her to be critical about Jewelius and his character. Besides, Jewelius already has on his plate the manipulation of the entire city of Canterlot, so he may not want to bother with tricking Twilight's parents whom he can declare to be casualties of the Changeling invasion, if he never has to force their daughter back into submission.
While interacting with Jewelius in the first chapters, Twilight doesn't act in the same grumpy way she acts while speaking with her friends about Chrysalis' behaviour. He's still a new acquaintance to her, and people are less likely to let loose around new faces to avoid making a bad impression, leading her to speak with him rationally about her concerns when he insists her to tell him about them. Though she has never met Jewelius before the first chapter, she has heard about him from his cousin Cadance who has also made her high opinion about him clear to Twilight. His kind treatment of Twilight, all the way down to his coup, reinforcers the impression Cadance has given about him. Since Cadance is the only fugitive who hasn't lost Twilight's love, the emotionally broken unicorn doesn't question her former foalsitter's words about Jewelius, blinding her to his faults until he stops pretending there's reason or kindness to him.
Jewelius' decision to have Celestia and Cadence killed, but send the Mane Five, Shining Armor, and Spike back into exile seems foolish on his part, until you realize that he could potentially prolong their suffering.
Being renounced by Twilight and witnessing the deaths of Celestia and Cadence would crush their spirits and eliminate any chance of trying to escape again.
If Jewelius found a more appealing unicorn mare and offed Twilight, he could've twisted the knife in Shining's heart further.
During the epilogue, Cadance says to Twilight that if the latter had given up on everypony close to her, none of them would be happily enjoying the wedding at the moment. It can also be played the other way around; none of them would be there if everypony had given up on Twilight, before or after her Jerkass Realization.
He also straight-out admits to Twilight's imprisoned loved ones that if he happens to discover a unicorn mare even more powerful than Twilight, he'll stage an accidental death for Twilight so that he's free to marry the other mare. The fifth season introduces Starlight Glimmer, a powerful and talented unicorn prodigy of her own right who can strip ponies of their cutie marks and enhance her magic with her emotions to the point that she can give a tough fight for Twilight when the latter is an alicorn. Plus, like Jewelius, she's good at using speeches, propaganda, and emotional manipulation to sway ponies to her beliefs and get them to join her. With that thought, the likelihood of Jewelius carrying out his threat may not be small after all if he found out about Starlight and considered her to be more appealing than Twilight
Jewelius murdered all the guards who were injured in the Changeling Invasion yet remained loyal to Celestia, then replaced them with criminals he had swear loyalty to him for their freedom. Even with those criminals being reimprisoned after Celestia is restored to the throne, Equestria is definitely going to have a military shortage for a while. Not to mention that the slain soldiers' families, who were already in grief over their deaths, now most likely know that they were killed by a sociopathic usurper.
Everything Ponyville's inhabitants have to suffer because of the raised taxes and trading prohibitions imposed by Jewelius. Mayor Mare tells that whenever anypony tries to resist these unfair laws, Jewelius sends his guards to do something she doesn't dare to explain further. Some ponies resort to theft in order to help the needy, and Mayor Mare hides them in the town hall to protect them from guards who could appear anytime to act on Jewelius' orders or cause trouble just for giggles. One of the ponies hiding in the town hall is Scootaloo.
Jewelius plans to create a new slave labor from the imprisoned Changelings. With their acts of nearly conquering Equestria and causing injuries to multiple ponies still fresh in the public's mind (not to mention the injured guards who were murdered on Jewelius' orders and are believed to be the Changelings' victims), some ponies might be tempted to accept Jewelius' idea to oppress the Changelings as revenge. At this point of the series, they're believed to be nothing more than love-sucking monsters, and it's not until the sixth season that they're proven to be misguided because of their selfish Queen's leadership.
When the escaped heroes attempt to rescue Cadance in the 13th chapter, they're stopped by Vivian's magic when they reach the medical wing. Jewelius then attempts to murder Cadance, planning to frame the "crazed traitors" for it, but he fails because Cadance is already free. What would have happened if he had succeeded? By that point, Twilight, filled with doubt because of Queen Chrysalis' words, is rushing to the medical wing. If she had stumbled upon that room to find Cadance dead, Jewelius would have probably acted despondent and regretful over banishing his cousin whom he couldn't save from the exiles. For extra cruelty, Vivian could have released the heroes before Twilight's arrival, allowing them to attack Jewelius in sorrowful anger for Twilight to witness. That could have pushed Twilight over the edge and convinced her that Celestia's apology speech and Chrysalis' words were all lies.