" 'the uniqueness of folk song' "
Without going into personal preferences, I think Bert's question summed it perfectly for me.
Extend his examples to music-hall, Victorian Parlout ballads, early pop songs... and there you have it
I'm not sure whether we can ever agree, but there you have mine
Incidentally - my opinion of 'hacks' is contained in the very word 2hack", the historical description of the output of these writers
Child used the less-diplomatic term 'dunghill' - if I am wrong, I am in good company, and proud to be

"An even more overwhelming majority of songs created by complete amateurs are even more unsingable."
And the unsingable ones never became folk songs as far as I know
"We know broadside songs were sung, because the street sellers sang them."
Not necessarily true Jack
Broadside expert Leslie Shepherd suggested that many of them were never sung - Pepys had a huge collection - I've never read thay he ever gave forth vocally
Isaac Walton described in his 'Compleat Angler' their use as ornaments to be pinned to walls.
We know as little about what was sung and what was not on the streets as we do about pre twentieth century traditional singing - very little.

Sorry - must go; the lady pipers are calling

"Can we tell anything from the style of language?"
Absolutely - the familiarity with vernacular usage says much about the songs as does anything else - as does folk humour
Jim Carroll