Chancing my arm at a theory I don't altogether subscribe to:
David Buchan, in his 'The Ballad and The Folk' put forward the suggestion that the ballads didn't have set texts, only plots, but, with the aid of conventions, commonplaces, repetitions, etc., the singer would re-create it each time it was sung.
I don't think he made his point very well, but like all such theories, I believe it to have a grain of truth.
Can a singer make a complicated song and commit it to his or her memory? I think so - as I've said, some of the singers we've met had phenomenal memories and they were steeped completely in their singing traditions.
It was once claimed (Lomax - Lloyd - can't remember), that in Yugoslavia a would-be bard would be apprenticed to a master and the leaving exam was that he/she would have to improvise a ballad of a given length to be accepted as a ballad singer.
Isn't that what Scots 'flytings' were - tests of improvisational skills?
Many of the Hebridean waulking songs were improvisations on a smaller scale (the one that was made on 'The Handsome American (Lomax) for instance).
In a small way I saw something similar with MacColl not long before he stopped performing. A couple of times his memory failed him (a very rare occurence) and he would improvise chunks of the ballad without it being noticed, except to those of us who knew his repertiore backwards.
Jim Carroll