Yeah, not all Sherlock Holmes stories are in the public domain. As it says in the case you linked only 10 stories out of 56 plus 4 novels were still under copyright (now down to 6).
The Doyle estate lost the case you're referring to and Klinger was allowed to publish his derivative stories.
Disney could try to sue, but I don't think their case is any stronger than the Sherlock Holmes case if the defendant is only deriving material from the original Mickey cartoon.
Klinger was seeking declaratory judgement that he could use the non-protected elements. Doyle estate was saying he couldn't. It was agreed by both parties that he couldn't use elements contained in the 10 stories.
From the case listed "And the claim is correct, for he acknowledges that those copyrights are valid and that the only copying he wants to include in his book is copying of the Holmes and Watson characters as they appear in the earlier stories and in the novels."
Mickey Mouse will most certainly not be in the public domain anymore than he is now. Disney can also fall back on trademark law to take care of Disney's use as well.
I think we have the same understanding of the case then. We seem to have a different conclusion on derivative works based on the original Mickey Mouse once it enters the public domain. I agree with you that modern Mickey would still be under copyright.
It's a bit of a moot point in my opinion, because I don't think anyone outside of Disney would even want to make more of 1928 Mickey. It's old. It's not popular. Maybe they'd use him for a cameo or for parody, but shows already do that anyway, regardless of the copyright [1].
The Doyle estate lost the case you're referring to and Klinger was allowed to publish his derivative stories.
Disney could try to sue, but I don't think their case is any stronger than the Sherlock Holmes case if the defendant is only deriving material from the original Mickey cartoon.