In Issue #2, Tempest Shadow expresses shock at the existence of "Nightmare Night". In the Friendship is Magic episode "Luna Eclipsed", Twilight asks Pipsqueak if he's having his first Nightmare Night ever since he moved from Trottingham to Ponyville, but he responds that is his first one at all. Considering that Tempest left Equestria approximately at Pipsqueak's age, she must have done it before she could ever participate in Nightmare Night, leaving her with few (if any) memories of the holiday to recall in adulthood.
In the same Issue, Stygian says that he didn't wanted to recruit Sunset Shimmer because he didn't wanted "another mirror" involved. However, there's another reason to not to try to recruit Sunset: Communication and time:
The idea was to keep things secret, so they couldn't ask Twilight to write her a message without explaining why. And even if she agreed without them explaining why (which is quite likely could have happened), Forgotten Friendship showed that there's no guarantee that she would respond as soon as the message was sent, and they couldn't waste time waiting for her.
That would leave another option: Crossing the portal. This brings two other problems:
A) If they crossed over the portal by themselves, they would have to adjust to their human forms in addition to search for Sunset. This could cost them a considerable amount of time.
B) If they went with Twilight to help them (of course, without telling her why), there would still be the possibility of Sunset not being in CHS at the moment, forcing them to search through out the town for her. Even if they split, Stygian and Princess Luna would have no idea of where they should search first. And, while Twilight does know where to search for Sunset, there's no guarantee that she can find her quickly, as she would still have no idea of where Sunset is, meaning that she could easily search at the wrong place. Adding to that is the possibility that Sunset may not even be on town at the moment, meaning that they would be on a wild goose chase. In short, all options are too risky to even try them.
Issue #3: If Trixie is in good terms with Twilight, then why is she angry at the idea of another Twilight taking her spotlight? Well, aside from Trixie being an Attention Whore, there may still be a few issues with Twilight she hasn't resolved yet. As Trixie points out, Twilight humilliated her. Humilliation is not really easy to forgive and forget, througth Trixie could have simply supressed her anger now that they are in good terms. But then, she meets this another Twilight who calls herself "The Great and Powerful Twily". Trixie may not take the idea of any Twilight using the same stage name as her that well. Add the supressed rage to that, and voala, the perfect chance to vent hidden anger (and some new one).
Why is Tempest so easily overpowered by Eris's hypnosis spell when she has nerves of steele? Well, looking at her characterization in the movie, she actually acts all calm when facing opposition and before she gets hypnotized, she also looks pretty calm when Eris comes to interrogate her. Being calm can set your mind at ease and due to this, her mind was easy-pickings for hypnosis.
Sunset Shimmer can be seen in Eris' casino. Thing is, even if she knew how to go there, she is not proud of her villanous past and is plagued with guilt for her actions, so she wouldn't like going to a place where she had to be evil or formerly evil to enter. Meaning this is another Sunset from a dimension were she never reformed in the first place But to what point is her story similar to the original Sunset's? Given how she is not a demon, this leaves three possibilities:
She never left Celestia in the first place, which could have serious implications for Twilight and Equestria as a whole.
She left Celestia, but never returned to steal Twilight's crown, meaning that she stayed as a bully in CHS and the human versions of Twilight's friends never reconcilied. And with the school still divided in groups and Bully Sunset still around, CHS' reputation may not have skyrockted (see the "Fridge Brilliance" section at My Little Pony: Equestria Girls – Friendship Games's Fridge page for more info), thus, Principal Cinch may not have had a reason to put Sci-Twi in Crystal Prep's team for the Friendship Games, meaning that Sci-Twi did entered the independent study program she wanted to enter, leaving her with no friends aside from Spike. And Crystal Prep is still in control of a self-centered principal who only sees her students as a way to maintain her legacy.
She did left and returned to steal Twilight's crown, but she refused to reform when Twilight gave her the chance, not only meaning she's still a bully but one who wants to take revenge on Twilight and her human friends. This could mean she began to bully even more Twilight's friends (and Flash Sentry as a way to indirectly attack Twilight). And when the Dazzlings showed up, there are two options on what happned:
She saw them as treats to her power upon realizing they are using Equestrian magic, and may have targeted them as well, but the Dazzlings either ignored her or used their magic to force the school to attack Sunset until she either left to save herself or was captured, meaning no one was able to stop the Dazzlings from taking Twilight's friends magic and take over the human world. And if she was captured, the Dazzlings may have taken her magic as well and released her once she was of no use to them. Leaving Sunset bitter and alone, with only her thirst for revenge against everyone to keep her going.
Sunset teamed-up with them, seeing working with them as her perfect chance for revenge against Twilight's friends, and may have even conviced them to attack Equestria in a Xanatos Gambit: If they succeed, Sunset may succesfully orchestrated her revenge against Twilight. If they fail and get captured, their spell only seemed to cause hate plagues, meaning that their conquest of the human world had to be througth force. And with the Dazzlings dissapeared and Sunset being the only ally they had, she could have replaced them as rulers of the human world.
The Mane-iac is among the villains in Eris' casino in Issue #1. Does this mean that there's indeed a dimension where the Power Ponies are real? And, how could have this possibly world have been affected when the Mane Six and Spike were transported to a Power Ponies comic?
The sole concept of alternative versions of Equestria where things went different can have some terrifying implications: What if there is an Equestria where Celestia became Daybraker but the Pony of Shadows wasn't able to control her? Or one where Twilight became a pony Midnight Sparkle? Or one where the Dazzlings never were banished to the human world?
Also, this could very well apply to the human world too. What if there is a world where Demon Sunset was never defeated? Or one where the Humane Six and Princess Twilight couldn't stop the Dazzlings even with Sunset by their side? Or one where Midnight Sparkle wasn't stopped? Or one where Gloriosa Daisy/Gaea Everfree won?
Additionally, what does this mean to the alternative timelines Twilight, Starlight Glimmer, and Spike created in the season 5 finale? Where they truly nullified or simply became alternative realities?
Among the villains in Eris' casino is a corrupted Rarity from My Little Pony: Friendship Is Magic (IDW) Issue 43 to 45. Given how Luna pointed out at the end of that arc that the obly way their corrupted selves wouldn't have been stopped is if they worked together, it implies that there's a dimension where that scenario did happened. Just, what exactly happened there? If they worked together, was that permanently or merely temporally and later turned their backs on each other? And if that happened, was that the start of a war between villains for Equestria?
Pony versions of Gloriossa Daisy and Wallflower Blush were at the casino. Given that is impossible they comited the same evil deeds their human selves did, just what did they did to be allowed to enter there?
