If you make a good effort to identify, locate, and contact copyright holders, but the path runs cold, can you disregard copyright? Maybe by claiming fair use or lack of traceable copyright?
Trademark requires active use. I don't believe there's such a thing for copyright. Are there limits other than regular fair use and documented year expiration?
It may depend on the country and state, but with a lack of heirs, it likely goes to the state like all other possessions. I'm no expert on this, though.