"IMO, the answer can only be 'maybe', because almost certainly these story songs were being sung before they were being printed."
Couldn't agree more - all we can say for certain about printed versions of songs and ballads is that they have been around at least since..... whenever.
"is quite entitled to seek out what constitutes the best version"
Again , agree wholeheartedly.
The problems arise when singers make claims beyond the evidence they have on the songs they have adapted - MacColl seldom did this (I can't remember him doing so) - Bert Lloyd did it all the time and left a minefield of problems in his wake.
Bert was one of the most entertaining speakers I have ever heard, but quite often his desire to entertain and impress made some of his statement quite unreliable.
With MacColl, the opposite was the case - he seldom divulged his own input into a song, but he made none of the claims of 'authenticity' that Bert was in the habit of making.
I remember sitting though a talk Bert once gave on Irish instrumental music and, even with my limited knowledge of the subject back then thinking, "hang on a minute..!!) - I enjoyed the talk immensely.
The difference between the two, it seems to me, is that Bert didn't always live up to the reputation he had built up for himself, while Ewan didn't always meet the reputation others had given him.
Aside from this, the quality of what we have been given by source singers and storytellers is often underrated.
We once recorded a magnificent long version of an Irish legendary tale from a Clare storyteller (The Gille Dacker and His Horse)
I knew it had been included in P W Joyce's 'Old Celtic Romances', and assumed that this is where it had come from.
When I mentioned that Joyce had published it, the storyteller replied, "I know, but he got it all wrong" - when we checked, he had!
I still find one of the most offensive statements ever made by a collector/academic was that made by Phillips Barry (who should have known better), to the song Lakes of Col Finn in the New Green Mountain Songster:

"Popular tradition, however, does not mean popular origin. In the case of our ballad, the underlying folklore is Irish de facto, but not de jure: the ballad is of Oriental and literary origin, and has sunk to the level of the "folk" which has the keeping of folklore. To put it in a single phrase, memory not invention, is the function of the folk."
Jim Carroll