"the talented broadside makers who composed so many of them."
The broadside "hacks" earned their title because they were poor poets working at great speed - the collections of their works show that pretty clearly
There is viritually no evidence as to which folk songs started on the presses and which were taken from already existing forms and rewritten for urban audiences   
The themes and structures of our folk songs, the insider knowledge and the use of vernacular suggest that most of them came from the communities they depicted
The question of literacy still needs to e discussed fully
Jim