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?

No. You don’t get to just decide you have the right to use someone else’s work just because you coudn’t find them to ask, any more than you get to decide that you can use their car. Them not actively selling their works isn’t the equivalent of leaving the car derilict on public property.