Mickey should not be in the public domain because the character has become a Trademark of Disney that represents the brand. Trademarks don't go into the public domain. While the story for Steamboat Willie will end up in the public domain, somebody couldn't make a copy of it and use the likeness of Mickey Mouse in it. They could use Ronald Rat and make a character that looks completely different and remake the movie. They could even call it Steamboat Willie.
King Kong doesn't represent any company. Therefore he was only protected by copyright and would therefore end up in the public domain.
I'd have to get a law degree to figure out something like The Peanuts. Does Snoopy or Charlie brown represent a company and qualify for Trademark? What about the other characters? Big Bird would probably qualify for a Trademark. Darth Vader, probably not.
Trademark law and copyright law are totally separate animals.
You can totally have a trademark that contains or references concepts, images, and prior works that are in the public domain.