Richard R: regarding instrumental music vs. song: I don't know much about that (being an unaccompanied ballad singer myself) but I found the following to be of interest and perhaps relevant:

(Background): A couple of years ago I came to realize that what I knew about folk songs, folklore, or ethnomusicology (as at least some would use the term) has been highly influenced by an "oralist" slant (my first teacher on the subject was an admirer of Walter J Ong). While still fascinated by the ideas about how primary or secondary or residual orality qualitatively influences culture and cultural transmission, I increasingly realized that the extent of literacy in Europe and the role of print (Broadsides) in the history of English language ballads had been somewhat underestimated by my earlier sources. So to educate myself more about that I paid a visit to the English Broadside Ballad Archive up at UC Santa Barbara (as well as the Wax Cylinder Archive at the same institution). This began a most fruitful conversation*.

Getting to my point, I learned that broadside ballad sheets almost never had a notated tune, instead simply stating "to the tune of...". At the Broadside Ballad Archive project they take a stand to use only tunes that were documented (notated) before 1701, and it turns out they get most of those from fiddle or dance tune books. This suggests to me that many folk instrumentalists in the 1600s read musical notation but singers generally did not? This makes me wonder if the role of aural/oral transmission in instrumental music would have had less influence than it has for songs.

PR

*Incidentally: if anyone is going to Camp New Harmony in Northern CA this winter I'll be holding a workshop to share (and sing) some of the broadsheet versions of ballads we also know from oral tradition.