"it's legitimate to simply assume an ancient and/or unique origin for folk songs that is distinct from printed sources. "
I assume nothing R
I believe that the only way to arrive at a conclusion lies in assessing what information we do have and bringing it together
The most important source has always been neglected - the singers themselves, but having said that, we do have a little from them and there is possibly more yet uncovered.
Roud and Gardham have discounted that information by turning the singers into customers rather than creative artists using their art to comment on their lives.
This is how Steve Gardham summed up folk songs in an earlier argument

Subject: RE: 'Historical' Ballads
From: Steve Gardham - PM
Date: 19 Apr 11 - 05:14 PM
"Yes, it certainly does place them on the same level as any pop songs churned out by today's music industry. They were the equivalent of POP songs when they hit the streets, and those that came out of the theatres and pleasure gardens and glee clubs and cellars in the towns were also pop songs. They only became folk songs when the folk started singing them. "
It really doesn't get more unequivocal that that - money rules OK
The two Steves views on this are inoperable, though Roud is far less arrogant and patronising in his declarations
Jim Carroll