copyright is automatic and implied, if someone somehow got a legal opportunity to see it, and managed to copy the work (or indeed if the publisher who originally refused it decided to publish it from their records), they would be breaking copyright from the original work regardless.

But once the copyright runs out then the publisher can publish it regardless, and a viewer could quietly copy it and publish it, unless someone in the estate made altered copies for anyone viewing it, then the altered copies would have fresh copyright, confusing the whole issue.

That's not how copyright works, no? Copyright exists when you write something, regardless of where you put it.

One hasn't had to file for copyright for a very long time. It's automatic now.