Going back to the original concern -- can there be an Ur-version of a ballad?

IMO, the answer can only be 'maybe', because almost certainly these story songs were being sung before they were being printed. (A cynical view of hack writers would have them feeding off existing stories/songs rather than creating new themes themselves.)

But with regard to recent comments about MacColl and his version of Sir Patrick Spens -- I firmly believe that any singer NOT brought up in the oral tradition is quite entitled to seek out what constitutes the best version (most complete or satisfactory) in his/her opinion: this may involve adding stanzas from other versions, or tidying up lines to make a more appealing listening experience.

(I'm one of several people -- Ronnie Clark and Gordeanna McCulloch-- who offer ballad workshops in which we actively encourage people to make their own choices about fitting text to tunes, and choosing verses from a selection of alternative texts.
So I'm not really in thrall to the notion of an Ur-text, but more than delighted with the grand proliferation of versions collected over centuries!)