Most of the songs here are very old. I can show you some ancient versions of 'Ten thousand Miles' for instance. If you alter the syntax and a few notes here and there anyone trying to claim copyright, if they found out and could be bothered, would have a job trying to prove your plagiarism.

What would be very useful is a thread on known cases where someone has been prosecuted for singing a traditional song, be it in someone else's arrangement or not.

I seem to remember when I first got interested in folk music in the early 60s there was a group of people offering to fight any legal case against anyone trying to effectively copyright traditional songs, and if my memory serves me rightly one of them was Bert Lloyd. A pity they aren't still around.