I can already feel that I'm going to get hate from some people about this, but I can't stand Rise of Red. And not in a "you don't like it because none of the OGs came back" type of way, but in the way that the story doesn't really make sense to me (because honestly, I couldn’t really care less that 99% of the OG cast didn’t return, that’s their right).
It isn't even really an issue with the present day characters (although I would've loved to have seen more Wonderland kids, especially since this movie introduced Wonderland so it would've been cool to have them present/mentioned, or even if we got the daughter of Alice teaming up with Red over the daughter of Cinderella), it's more of an issue with how they handled the past.
Having all the parents go to school with one another at Merlin Academy (which has no possible way of existing in the canon without retconning Ben's line in D1 about how Beast turned the building into a school after he became King) made little to no sense, and it honestly left me with more questions than answers at the end. Questions which will most likely never get answered now that Wicked Wonderland isn't going to take us back to the past like the original D5 plot was meant to.
On top of that, Rise of Red basically retcons the entire premise set up in the first three movies that all the fairy tales happened the same way they do in the OG animated movies. None of that could be possible by Rise of Red's standards because everyone already knew one another or were in relationships with each other before their stories even took place. If we're to believe all the stories still happen in the same exact way here, then Aladdin and Jasmine wouldn't be together yet, and Charming and Ella would never interact until the Ball (among other things).
All-in-all, for what it’s worth, I do think that if Rise of Red had been the first ever installment in the Descendants franchise, I might’ve actually enjoyed the story it was trying to tell, but I just sadly wasn’t able to. And don’t get me wrong, I have issues with the other movies as well, but there’s just something about Rise of Red that I just can’t get behind.
Again, I can feel I'll probably get some hate from some people in the fandom over this, but I've been wanting to vent about this for way too long so I figured why not.
Your observation about the Merlin Academy being a retcon of an iconic line from the first movie is spot on.
In Descendants, Ben explicitly tells Mal and the VKs that the school building they are attending, Auradon Prep, was created by his father: "My dad, he spent all of his time focused on turning the old castle into Auradon Prep." This line firmly established that King Beast founded Auradon Prep using an old structure, implying it wasn't a pre-existing institution for royal children.
The shift in the Descendants 5 (now Wicked Wonderland) plot further emphasizes the disappointment of this retcon, as it leaves many crucial questions about the past unanswered:
Why did Merlin Academy close? If it was the primary school for Auradon's heroes, what caused its closure, and why did it stand empty as an "old castle" until Beast reopened it?
The Villain Kids (VKs): Were all the future VK parents present at Merlin Academy? Did they all interact before the Isle was created? The film offers a glimpse, but a comprehensive history is missing.
Beast's Motive: Why did Beast completely remove the name "Merlin Academy" if it was a prestigious school? The original line suggested a creation rather than a revival.