Nonsensical. There are several transformations mods that just work. Also, Agumon>Greymon isn't even a mod (anymore), it's in core game, and the transformation works.
Zelda has no publically available sprites, that's why she's not ingame. It's a lot of work to create a spriteset from scratch, and they simply didn't want to. But it has nothing to to with an engine restriction.
Viewing post in Super Smash Bros. Crusade comments
you could switch between the playstyles on the fly as link, but that miiiight've been a mod thing though. so it counts but not really.
also. poketrainer not being zelda is a very shitty argument.
you asked me for characters that can switch with a single input, I named characters that can switch with a single input. so what's wrong now, huh?
edit: cmc+ having a different engine doesn't matter. professor oak, an april fools character made as a joke, worked in 9.2's version of the engine, and it worked PERFECTLY FINE. 0 HITCHES WHATSOEVER.
edit2: hell, I'll even go out on a whim here. out of every character I listed, only *one* wouldn't really work in 9.5 and it's vent/aile. they're too much of a technological achievement that they really, REALLY don't feel compatible with 9.5. every other character, including the shiruzato link which came out 5 years ago, have genuinely zero reason to shit themselves in vanilla outside of being v8 exclusive or something stupid like that.
And so does many existing mods. Some even have more than two transformations : Vent/Aile not only has two entire characters in the same mod, but both have like five different armors with entire movesets they can switch at any time.
Also the number of different moves don't matter, it's all the same code structure.