Well, I mean, to at least the die-hard fans, they'd sell like hotcakes, but I do see your point. And as far as KH cloud goes, I think that makes sense; Dissi is a Final Fantasy game, and while KH is a Squeenix game, it wasn't part of it to begin with, so why put it in the second? I like the outfit idea though.
Probably not, considering that the KH team wouldn't need to do any work in either case. I can't really say why they didn't want KH Cloud in Dissidia, unless they wanted to keep the KH versions of FF characters out of Dissidia for some reason. The Final Mixes would only take translation, no porting, the only likely reasons that we don't get the FMs is because they are made to be Japanese exclusives, like the International versions of the FFs. Another possible reason could be Square-Enix not believing that the FMs would sell well enough in other countries after releasing the initial versions to justify translating them.