Agreed. As I've said many times literary ballads often go into oral tradition pretty quickly. Good example is 'Bonny Lass of Fyvie'.

I'm not really qualified to comment on jazz pieces but I've never heard of anyone trying to separate Elfin Knight from Scarborough Fair before. The loss of the supernatural elements is a common occurrence at this period and the refrains jumped around from ballad to ballad willy-nilly. For instance there is cross-over between refrains of Child 277 Wife wrapt, and Robin-a-Thrash, but we now know these 2 ballads had separate origins and evolutions.