Just to clear, by "folk songs" here, is Roud referring to songs he's given a Roud Index Number to – all the folk songs he's ever come across? Is he saying 90–95% of the folk songs he's ever encountered have appeared in street literature?

He means 'folk songs' as notated by collectors in late 19th / early 20th century England.

As to the other evidence, I'll let you finish the chapter rather than try to paraphrase. However I don't think SR would dispute that broadside writers were quite capable of plagiarising traditional songs as well as other people's broadsides.

'Streams of Lovely Nancy' is an interesting one. As my old friend Roy Harris once wrote: "One of the loveliest jumbles in English folk song. Impossible (so far) to know what it's all about." But then, the song as sung in the revival didn't always include all available verses.

I had a quick look at the Bodleian site, where there are loads of SOLN broadsides, with at least two different versions of the story (such as it is). One follows the standard opening with verses about a woman parting from her sailor lover, while in another the opening is the same, but he seems to be a soldier judging by the reference to 'marching away'. The place names change as well. What that tells me is that at least one and possibly both of these are rewrites of another text, but that the writer wasn't particularly worried that the opening verses didn't make much sense or have anything to do with the tale of the parted lovers. Here are two examples:

'Streams of Lovely Nancy' (1)

'Streams of Lovely Nancy' (2)