"If the author is known, it is not trad, even if the author died 70 years ago."

I'd agree in the main, Peter - because I don't think we can now add to that body of work that was created in another age - the world has moved on, and the processes no longer function.

But we have to remember that we do actually know the names of the makers of quite a few of the very very old songs, works which no-one would dispute are now traditional. So this is a grey area in some respects.

I was wanting to stress that for the lifetime of the composer, and 70 years thereafter, no-one can even START to think of the song as trad, (however 'tradish' is sounds, or however 'traditionalised' it has become in usage), and even THEN it would probably not qualify as traditional by most people's definitions - though there would be room for some debate at least.

We need new words!

"If Seth and his record label subsequently decide to take complete ownership of the song, will other people who may have recorded the song, performed the song at a paid for engagement or used it in some broadcast, play or presentation have to pay retrospective royalties?"

I think that's very unlikely in the circumstances, but it would make no difference to anyone who plays or records it. Seth would just get a bit more from PRS.

Agree completely with the rest of your post.