I think we're at cross purposes again, Jim.
You seem to be referring to the big ballads in your last statement. We were discussing folk song in general most of which are ballads. However, since you refer to commonplaces, these are also extremely common on broadsides.

'the broadsides were largely unsingable doggerel' I don't know about 'unsingable' but you are right, the vast majority of it most of us would consider extremely unfashionable today. But to those of us who have trawled through mountains of this stuff in order to track down earlier versions of 'folk ballads' we have managed to arrive at some reasonable conclusions about their origins and reworking. If you read your Mayhew and similar you will know that in previous centuries recycling was a massive industry unlike today's throwaway society.

Matt, if you want I can't point you at some academic works that are very useful on some of the points that you mention, but I don't have a comprehensive list as I'm not an academic.