What I can't swallow is the folk song as seamless evolution. I believe it's the unique creation of a talented individual, sometimes 2 or 3 such people with a dash of synchronicity, certainly tweaked and patched up, but decidedly not an incrementally growing artefact of 'the people'. Whoever the hell they might be.

"In Music Halls it's reported an audience could remember an entire song on a single hearing"

A very good point. My father's literacy levels were what you'd expect of a chap who left a small rural school at 14 but his recall of lengthy poems far exceeds my own - M.A. and all. Rote was the tool of choice for retaining all manner of information and un-literate people became expert at commiting to memory.