Scrooge never lets his number one dime out of his sight, unless it needs to go into an even more secure vault. As it turns out, part of the reason for this is because he has his most dangerous enemy imprisoned in it. Truly a case of keeping his friends close but his enemies closer.
Magica's use of Buffy Speak makes sense given that she likely picked up a lot of slang while being Lena's shadow for 15 years.
Scrooge is essentially Lena's father, though not biologically. Even though there was no natural/traditional procreation of a duckling involved (as there is absolutely no love lost between Scrooge and Magica), and Lena is instead the result of magical processes, "born" as the 15-year-old we meet in the series, Scrooge's actions were nonetheless the trigger for Lena's creation. He may not have contributed his DNA, but he did ultimately cause Lena to exist. Thus Lena already was and is a part of Scrooge's family, even without her current connection to Webby.
When Donald realizes what the Barksian Voice Modulator does, the first thing he tries saying is "rubber baby buggy bumpers". Three times fast, no less. It seems a pretty random (if difficult for someone with Donald's normal speech impediment) thing to say until you remember how overprotective Donald is of the triplets. Rubber baby buggy bumpers is probably something he actually tried to get at some point.
During his attempt at a Darkwing Duck theme, Launchpad laments that he can't think of anything to rhyme with "fight". Launchpad is fighting shadows. What both casts and dispells shadows? Light.
Donald's head has long been established in this series and the old comics to be incredibly hard, hard enough to damage stone when he was a child (the comics) and break through a magical pachinko machine (this series). Hence why Magica's amulet breaks upon hitting him in the head.
How was Della able to survive being on the moon for over a decade? The cabin of the Spear of Selene likely would have been strong enough to survive the crash intact, leaving her with a decent shelter, and stable oxygen chamber, and depending on how soon after they would have went on a space trip, Scrooge could have already had supplies, including food, water, and oxygen, already on board for three adult and three infant/toddler ducks for anywhere from a few weeks to a year or more, plenty for a single duck, especially if she rationed them.
Scrooge promised Lena that she would be a part of his family if she helped him stop Magica and get his family back. She did both, and was apparently destroyed for it, meaning that Scrooge apparently can't keep his promise... apparently being the key word. Lena's merging with Webby's shadow means that she is a part of the family now, even if they never find out she's still there (and let's be honest: what are the odds that they will never find out?)
When the kids are invited for a midnight swim by Scrooge, most of them are just rolling around on the surface of the coins, but Louie can dive and swim in it just like Scrooge. After drudging through the coins all the way back in "The Great Dime Chase!" to find the decoy dime, he probably learned the essential skills to do so.
In their battle with Magica, Huey and Louie only really get two hits against the shadow sorceress before retreating to the background. Why? There's some good reasons both in and out of universe.
In universe, Webby and Dewey were FAR more comfortable with going against a physical challenge compared with the bookish Huey and the smooth but lazy Louie. So they're going to be the best for going after the dime directly.
Out of universe, it makes the most sense because of arcs. While also upset and involved in different ways this season, Webby is personally upset at Magica for trying to destroy her family through her best friend, and Dewey had the biggest connection to Della and the drama with Scrooge before the attack. So they're going to be the main ones to save Scrooge.
When fighting Magica, all four kids use what they are best at: Huey distracts her with questions regarding how her magic works, Louie tricks her into believing that the mirror she destroyed is a cursed artifact, Webby gives her a No-Holds-Barred Beatdown thanks to the combat training she got from Mrs. Beakley, and Dewey courageously takes the Number One Dime from her and frees Scrooge from it.
Gyro says that his assistant that beats the most shadows earns his respect, and the other one gets fired. But since Fenton turns Li'l Bulb into a gun and rides Manny while fighting the shadows, Gyro's assistants all play an equal role in defeating the shadows - therefore Gyro can't fire either of them. Maybe it was all a test of teamwork?
Della being revealed to be alive makes sense when you take into account what happened near the end of "Secrets Of Castle McDuck". Many of the dead relatives scare off the demon dog that antagonized Huey, Dewey, and Louie, with the latter three being none the wiser. Della is not seen among them, meaning that she never died.
Lena's hideout is pretty much tailor made to show to Webby and the viewers that Magica's later claims about her not being a real person are false; it shows that she has physical needs with the bed, the food and the toilet paper, and the posters on the walls make it clear that her personality is real, as well — not news to the viewers who've seen her inner monologue, but enough food for thought for Webby that she doesn't fall to complete despair over Magica's gloating.
When Magica has Gyro hostage, she asks the captive Scrooge to confirm if he's a "good friend, or work acquaintance." When he confirms the latter, Magica tosses him aside (off a cliff, mind you but he does survive no worse for wear). While it seems like a Kick the Dog moment for Scrooge, keep in mind that he probably realized that if he'd confirmed the former, Magica might have tortured Gyro in front of him. Denying a close connection had the best chance of either in either her sparing Gyro, or at worse a quick death, thinking him of little importance.
During the fight against Magica - who has been hijacking the shadows of all Duckburg with her dark magic - a completely untrained Webby manages to hijack Magica's shadow with the Friendhip Magic last seen in Jaw$. How? Because Magica's shadow is Lena, Webby's best friend.
Donald's plan involves attacking Magica from every available environment: air (Launchpad), land (Gyro's team), water (Donald and Beakley), and underwater (the kids, though they didn't know they were part of the plan).
Why does Donald need to employ Reverse Psychology to get the kids to the Money Bin, when they were willing to go anyway?
Donald is literally the personification of bad luck. He probably knows that any plan he makes is doomed to failure. He told Launchpad to crash, and that managed to fail. By separating them from him, their actions fall into a sort of in-universe Unspoken Plan Guarantee- he's not part of it, he doesn't know about it, so his bad luck can't screw it up, and if his bad luck does stop him from getting to the Bin, it's not going to stop the kids. He's essentially making himself the distraction, while still giving himself the option to be backup for them, just in case he does make it through.
Donald hasn't been adventuring with the kids. He knows they have skills that would be useful, but he doesn't specifically know what they are. While he knows their personality traits, he's not going to be able to come up with a better plan to utilize them than they could come up with themselves.
While Donald does gain the ability to focus his rage into protecting the boys (hence his hypercompetence in this scenario), he'd probably circle back to berseker-mode if he actually saw the them being threatened. He trusts them to be able to make it to the Bin, but he also knows that he wouldn't be able to do it if he were with them, which would jeopardize everything.
One of Launchpad's classic boasts is that he can (safely) crash anything. Apparently, that also includes "crashing" himself into Magica.
Fridge Horror
It's revealed that Lena is the shadow of Magica in living form. She was created during Scrooge and Magica's battle on Mt. Vesuvius 15 years prior to the events of the series, as a result of Scrooge trapping Magica in his number one dime. Lena is her own physical entity, with her own will and personality, and exists just fine alongside Magica's physical form. Since she is this, and was produced from an extension of Magica, she is, in all relevant senses of the term, Magica's daughter. The titles of aunt and niece were likely established so that neither Magica nor Lena could become emotionally invested in their relationship, yet Magica could maintain authority and control over Lena. This makes Magica a literal Abusive Parent.
In season 3 it's revealed that Magica has a brother, Poe, whom she loved dearly (and likely the only person she showed any affection to). Unfortunately, Scrooge was the reason Poe is gone, and it was so traumatic that she swore a blood vengeance on him. Naming Lena her Niece can either be her twisted way of showing affection to her lost brother, or it could also be her way of distancing herself from Lena so she can never feel the same kind of loss again. Or even more messed up, it's her way of spiting Scrooge; Lena was born from the pain and hatred Scrooge caused her by taking away Poe, making Lena Scrooge's daughter in Magica's mind as a way to spit on him valuing the idea of "family".
Now we know Della is on Earth's moon, a lot closer than we thought, and thus the last place Scrooge would've thought she would've ended up. The other rocket-men? They all went too far into space looking for her, not only not finding anything, but could not find their way back to tell the tale.
It’s revealed that Magica made Lena out of her shadow 15 years ago. And considering how Magica treated Lena, think about how she suffered Magica’s abuse for that long.
Whilst Donald getting his trademark voice removed for this episode is played for a gag, it basically establishes that Donald's vocal chords are legitimately disabled In-Universe, and that even his own family members find him The Unintelligible. On it's own, that's closer to Fridge Sadness, but then you have to ask yourself: if the rest of Donald's family speaks normally, why does Donald quack and splutter like that? Did he injure his throat somehow? Given the dangerous lifestyle he lived as Scrooge's ward, there's all kinds of nasty ways that he could have picked up his voice...
As seen in a later flashback episode, Donald's had his quacky voice since childhood. So it's either something he was born with...or the incident that caused his vocal change happened at a very young age.
