With regards to the inclusion of phrases such as "Like she whom the prize of Mount Ida had won, There approached a fair lassie as bright as the sun": surely one possibility (and of course not the only possibility) is that a non-literate writer of the song had heard this line elsewhere, thought it fitted a song s/he was writing and/or perhaps liked it and included it in their own. If the rest of the verses in that song don't sound "broadside" the perhaps they aren't.
I have done the same thing myself when I have occasionally found myself making up songs. I don't record or publish these, just sing them down at the pub so I'm more concerned about whether I like them (and how well, or more likely badly, they go down when I've sung them) than exactly how I come to make them up. In this way, perhaps I resemble some of those songwriters that existed in the illiterate classes of the past. (I know quite a few others who write songs like this - perhaps non-commercial "folk???" songs are still being born after all but who wants to collect them until or unless they've survived for a few generations?!) When such songs come to me I sometimes find I've inserted a phrase that's been inspired, or even lifted complete, from some other song. It never more than just a turn of phrase so I'm not concerned that I'm infringing copyright (and anyway most of my other repertoire and source of these phrases is "traditional") so there they sit in "my" song. Surely that's a possible explanation for some of the occurrences of broadside-like phrases in songs that may not have been entirely composed by broadside writers.
By now, my reading of the posts here seems to suggest that just about everybody is accepting songs arise from a variety of sources and sometimes from a mixture within one song. Agreement seems to have been reached on this but there are many who have fallen to defending their own views that no-one is really attacking as completely wrong anymore (if they ever were) simply because they have failed to notice no-one is totally disagreeing anymore. The world isn't black and white; it's grey.