The Firebird isn't evilIt's just a nature spirit responsible for destruction. Note how he just burned the trees and not killed the elk.
The Firebird will appear in Kingdom HeartsWishful thinking yes, but Chernabog was a random boss fight and a lot of people enjoyed it. Though if the bird was a character in the game, I doubt a boss fight would really fit the mood, more of a race against time ordeal.
The Devil from Fantasia is the Bigger Bad of the entire Disney multiverse and commands all the other villainsIt is he who whispers in the ears of villains like Jafar, Ursula, Scar, Cruella de Vil, Frollo, Maleficent and the others, inspiring them to commit their evil deeds. And they don't even know he's doing it. He's that subtle.
The Sorcerer's Apprentice was All Just a Dream.Either that or Mickey Mouse had an Acid Reflux Nightmare.
Chernabog and the Firebird are the same beingMore specifically this being living in a mountain is a variation of Gods Need Prayer Badly which is based on whatever the residents fear. The first incarnation years in the past is a satanic being repelled by prayer while the second is influenced by contemporary fears of deforestation and fights the dominant earth deity. Supported by the locations appearing to be the same with the ending shot of fantasia through the trees being reversed in the ending shot of Fantasia 2000 backing up through the trees.
Yen Sid was putting his apprentice through a magical equivalent of Wax On, Wax Off.Yen Sid wanted the apprentice to use magic to speed up the task. It was a test. A test he failed.
The two birds in the "Claire de Lune" segment are The Admiral And The Princess.