Unfortunately I came late to this thread, so I'll offer just these opinions for discussion.

Except in an extraordinary case, forensic proof of whether the ur-text of an old song was written by a broadside hack, a plowboy, or even by committee will never be forthcoming.

The more coherent, impersonal, and detailed a traditional narrative is, the more likely it was created by someone who was literate. Lyrics made up largely of "floaters" are more likely the work of unlettered people because they're less used to learned requirements for coherence and consistency.   

First-person narratives with many atypical deatails are more likely to have been written by a participant or close observer.

It would be extremely interesting to compare, stylistically, a corpus of songs never found on broadsides with a somehow comparable group that we know made early broadside appearances.

Finally, as has been said, "trad" songs, no matter how modernized in performance even by geniuses like the early Steeleye Span, are no longer the "music of the people." They are the music of enthusiasts like us.

Trad dance music is a little more popular (square dances and so on), but overall the teeming millions want their rock, rap, country, classical, etc. The eclipse of trad music is sad, perhaps, but inescapable. Cf. my note yesterday on the "Drop Everything" thread, suggesting that even for Mudcatters, trad isn't all that exciting.

Or is it just the weather here?