Jim, first my apologies if I have oversimplified or misinterpreted your argument but it appeared to me that your justification for claiming that 'Banks of Sweet Primroses' originated from the folk rather than a professional composer was based on the style and quality of the text.

"It is the overall style of broadside writing and their one dimensional approach to their subject matter that makes them unsingable
Broadside style is as identifiable as folk song style - you know one when you see/hear one"

That's certainly true to modern ears, but the fact remains that these songs were widely taken up and sung. The need to lick them into a more acceptable shape doesn't seem to have deterred singers at the time. However, while the words may have been taken from a broadside the singer would probably have heard the song first from the ballad-seller. In your own words, it's not until you put a song in your mouth as a singer that they spring to life. Those street singers whose livelihoods depended on people buying their ballad-sheets were probably skilled at making these unpromising texts appear attractive. The folk process then took over.