"all part of the creative process."
Agree Steve, but not part of the tradition (whoops!!!)
"the responsibilities of the collector to his informant"
In our experience, it doesn't really matter to them one way or another.
Every single one of them we questioned said (in so many words) that they considered themselves 'storytellers' whose stories came with tunes.
A lot of singers we recorded learned their songs from 'ballad sheets' bought at the horse or cattle fairs with no tunes indicated, so they put their own to them.
They had two distinct attitudes to the printed word - either to treat it as writ in stone or to use it as an aide-memoir and chop and change it as they saw fit.
Oddly, it was the non-literate Travellers who stressed the need for accuracy.
The common complaint of the ones we met was that "the young ones coming up don't seem to make sense of the stories - they don't mean nothing any more".
I think that, when putting the songs into print, either faithfulness to what you have been given is essential or, if you make changes because of memory lapses, you need to indicate that you have done so and explain.   
End justifying means is certainly not a major feature of Stalinism any more than many other political (or cultural) philosophies.
MacColl's 'Stalinism' was misleading in the sense that, when he was growing up the Soviet Union was believed to be to only workers state - some of my family shared his view - it was a generation thing.
He was, for a time, an admirer of Mao, but I once saw him go spare when the Chinese Government issued an edict on "the bourgeois nature of Stanivlaski's 'Method'".
MacColl's attitude to his work was a class rather than a political one, though he was happy to use it for political and social causes such as C.N.D., the Anti-Vietnam movement and Anti-Apartheid.
I honestly believe that Bert's approach was more or less the same, though he wan nowhere near as active.
John - would agree with most you say as long as creating songs in the folk idiom, especially on historical subjects, doesn't end up with writing pastiche - which is a tendency.
Jim Carroll