Jim - you have a wonderful record of collecting songs - all in the later 20th century. Must - if not all - of Steve's theories relate to the collections made in the early 20th century, IMHO a significantly different period.
I have actually not read Steve's book - but I have been to one of his presentations of the contains - that was very specific (however I don't have notes), and I found it very appealing.
I too have been studying the same period and I too have found examples of the existence of broadside versions of songs collected by Gardiner in particular.
Personally - I am interested in singing the songs and who sang them - not their origins - but if I find a connection to a broadside, I have to assume something......

I wish you well with your op, and I am sorry if you thought my comments insulting - but I hope you do glean something from Steve's book - it is 700 pages long, and he has been working on it for a very long time and has significant knowledge - so there is probably some truth between the covers.....

Tim Radford