That would make relay logic more complex and require tracking post-modification time, plus syncing that across multiple relays can be a challenge. But it is possible.

Yes, but this seems like something that needs to be solved on a protocol level, not by clients. If only some do it, then other clients wouldn't know what to do with your edited post.

No. Doesn't the ability to edit remove one of the safeguards against censorship? The ability to be strong armed into retracting statements, removing or changing information becomes possible.