It's risky to generalise on whether or not traditional singers deliberately altered songs
Some did - Yorkshire singer Arthur Wood did a brilliant remake of The Tailor's Britches (or his father did - can't find my note to it).
On the other hand, many singers we met insisted on both the 'correct' words of a song and 'the right way to sing it.'
"However I think there must be a strong chance that at least most of them were written by these authors."
On what grounds do you come to this conclusion Steve?
Sorry to persist with this but, as I have asked on several occasions and received no reply - if you concede that Aberdeenshire farmworkers were the main creators of their repertoire - why not sailors, mill workers English rural labourers et al?
American folklorist, Duncan Emrich once wrote "memory not invention is the function of the folk."
Your earlier arguments regarding the broadside origins of our traditional repertoire, coupled with your latest bombshell: "I go further and would state generally that where widely differing versions are found this is not down to oral tradition, but down to rewriting by broadside hacks", seems to indicate that you not only agree with Emrich, but go further to deny the existance of a folk process altogether. Why cannot widely differing versions simply be the outcome of time/distance/differing values, vernacular, customs... etc from differing communities?
It is our experience that along with a healthy song tradition, the communities we worked with also created songs. There is no reason why what we recognise as our national repertoire is not made up of locally-created songs brought out of the home communities and being absorbed into the larger repertoire - can give you plenty of examples of same if you wish.
One of the great Ulster singers was once asked by a relative going to work at the tattie-howkin' in Scotland would she like something brought back. Her reply - bring me back a song. The relative obliged and the song became part of her Northern Irish repertoire.
You haven't begun to address the question of literacy (or lack of same) - the fact that the communities that provided us with some of the greatest and rarest ballads were totally non-literate (the Travellers) and that even in the communities that had access to education, the acceptance of songs in print was not at all straightforward. The misgivings toward written song texts displayed by James Hoggs mother in her conversation with Walter Scott persisted among source singers right into the mid-twentieth century, certainly in rural Ireland.
For the life of me I can see no reason why an author's name attached to a broadside could not be the same as seeing a traditional song headed "Trad. arr. Fred Bloggs" - as you said yourself, they were doing it to earn a living.
Jim Carroll