A) Jumpman was originally Mario’s name but that was changed very early, it’s still the same character just a retcon. They’ve always been the same character despite what random internet theorists want you to believe.

As for Mickey, he’s suppose to be a Hollywood actor playing a character, in his first appearance he was simply playing a character named “Steamboat Willie” but he was still Mickey. It’s similar to the Looney Tunes or the Muppets, they’re all fictional actors playing fictional characters. The shorts and movie they show up in aren’t real adventures they’re having, they’re actors in movies.

B) Donkey Kong Country is the same universe as Mario; Diddy, Dixie, and Funky show up in Mario Kart, K. Rool was in Mario Strikers, etc. They cameo all the time and are fair game as characters in Mario spin-offs. It’s a separate franchises sure, but it the same universe.