.... and/or files a trademark application, Mickey Mouse will enter the public domain.
IP law doesn't work that way. You can either patent, copyright, or trademark. It's mutually exclusive. Once the copyright expires, they're done for and is the reason why they extended the copyright terms in the first place. Trademarks don't have expiration dates as long as the company is using the mark and continues to pay the registration fees (that also go on for eternity).