much of the more drastic variation is down to rewriting by the broadside ballad writers

A few years ago I researched the history of'The Wild Rover', helped along the way by both Jim and Steve G amongst others. Like a lot of older broadside pieces it began life as a very wordy, moralistic, thirteen-verse text written in the late seventeenth century by a known author (and yes, I do think this copy is most likely the origin). From there it went through various print incarnations over two centuries, getting shorter, more concise and telling a more effective story with each new edit. However, during the later history of the song it was also changed through what I could only conclude was oral traditional processing as well (the two are not of course incompatible), and acquired at least three distinct tunes, including a particularly attractive one in Ireland (as sung by Pat Usher) that made its way to Australia - where the song must have arrived independently several times. The well-known version is definitely a 1960s concoction, though.

That's just one example of how different kinds of process can affect the evolution of one song. It doesn't have to be 'one or the other', and we don't need to take to the barricades about it.

Steve Roud's book mentions the nice example of a rather arty song, 'The Shepard Adonis' becoming the localized 'Shepherd of the Downs' in the repertoire of the Copper Family. We don't know how or when the change took place but, since it appears there isn't a broadside copy of the 'Downs' text, and oral versions are vanishingly scarce, perhaps it was indeed a Sussex countryman who amended it?