For the Mickey stuff...
When Conan O'Brien was fired, his leaving contract stated he couldn't appear on TV for six months. I think something similar is in place here.
Either Wayne Allwine, Russie Taylor (Wayne Allwine's wife and the voice of Minnie), or Brett Iwan (Bill Farmer or Tony Anselmo are also candidates, though not likely) could have it in their contracts that Brett can only be officially recognized as Mickey after X amount of time passes after (something. Wayne's death, his official casting, etc.) Keep in mind the only bits we've seen of Brett is in commercials, stuff at Disney World, and the short version of the shorts. None of these have credits.
Bret Iwan himself said he was voicing Mickey in this game.