"in fact he wrote piano accompaniments"
No no, don't go there. He wrote piano accompaniments so the songs could be reused in middle class social gatherings where singing was generally expected to be accompanied. I don't think we know if RVW thought piano or other accompaniments were part of folk song in its natural habitat.

Nevertheless, there is no reason why instrumental accompaniment might not have been used in the past, as by the troubadors.

The fact remains that a good yardstick for a folk song is that it stands on its own without needing instrumental arrangement. If it can't survive being passed on as an unaccompanied song it won't live long. Many pop songs fail this test, which in my view distinguishes them from modern folk songs.

Anahata