I do have a comment on the book. It may be I missed something because I was not thinking of it at the time, but I don't recall a discussion of the relevance of solo versus communal singing on the development of songs.

It occurred to me when reading the recent 'Lord Randall' thread. That strikes me as being a very 'robust' song suitable for a serious solo, or with some joining in on the repeated parts, or as something like call and response maybe with boozed-up wags sometimes throwing in jokey substitutions. I first came across it 'communally' as the vestigial (parody or creative abstraction?) 'Green and yeller' as sung by Pete Seeger but maybe created for the barrack room.

Roud does mention burlesques and parodies comming back into the oral tradition not recognized as such. How much of the shortening down to a few versus that Walter Pardon comments on for Music Hall also went on amongst the folk to allow something that most people could remember and join in with. I have heard it said that one characteristic of the style of the Music Hall was that people sang what they had heard whilst walking home and looked forward to the next time.