Steve,
I am referring to "the corpus of traditional songs common to the British isles, North America and a few other English-speaking parts of the world."
We have no evidence whatsoever that the songs originated from the broadside presses, let alone made their way back to them - or if we have I am unaware of it.
There have been various claims down the centuries of authorship of , say Barbara Allen (about a century after Pepys referred to it as "That old Scotch song), but I believe these to be spurious - totally lacking evidence.
I was fascinated by Bronson's essay on the ballad Edward, though the only thing it convinced me of was that you should stick at what you are good at.
If you are prepared to concede that the bothie workers produced their own songs, why not the sailors, farm labourers, mill workers.... and the rest of them.
You are right we can't name the vast majority of authors - so on what do you base your claim that they originated as broadsides?
I would like to see your list of claims to authorship, but I would expect to see the evidence that goes along with it.
Looking through the broadside collections: Bagford, Roxborough, Ashton, Euing, Pepys...... even the later ones, Henderson, Holloway and Black.... I am always struck by the differences in both style and content to the traditional songs - how unsingable they are - not the similarities.
Jim Carroll