The word that is missing from this whole thread is "respect".
I learnt my craft in the folk scene in Scotland and was soon informed if I was taking liberties with a song. I learnt to RESPECT the song to RESPECT the melody, and RESPECT the composer. Is that too much to ask?
Traditional songs were not written by a committee they didn't suddenly appear out of the blue, each one was composed by one person. The fact that we do not know the attribution of any given song is no reason to treat the performance of that song as an excuse for a cavalier free-for all. It behoves us all to perform the song as the original composer would have wished . That is the criterion.
Respect. Not a dirty word, is it?