"There will be those who know more than I do, but is it not suspected that Mrs Anna Gordon may have played a part in 'creating' some of the ballads that she reported and which were published by Child?"

To answer my own question: yes this has been suspected.

For those who have not heard of her, Mrs Gordon (1747-1810) was a Scottish woman, married to a professor of humanity/army chaplain/minister.   

She wrote down a lot of ballads. Some were published by Scott and Jamieson. Wiki says that 27 of the A texts in Child's EWSPB came from her.

I have come across several pieces referring to her. For example, there is a whole chapter in Fowler's Literary History of the Popular Ballad.

Controversy over how to position her has existed for a very long time, as far back as Scott and Ritson. Attempts to argue that questioning this demonstrate something contagious emanating from the 20th century work of Dave Harker would, for me, serve merely to undermine the credibility of those advancing such a view.   

However, it could be argued that she was not so much a singer as a collector. On that basis maybe a better example could be found. But on the other hand, people do refer to her 'repertoire' so it seems that she is sometimes regarded as a 'singer'.

Another problem here is that often what singers have said is presented to us in a mediated form, and sometimes by mediators who strongly hold particular ideological viewpoints about folk in general. So we get double layers of 'mediation', or, if you prefer it, 'interpretation'.

Another is that not all those seeking the views of singers have been interested in tunes. Indeed, some downplay the importance of the musical side altogether. I think this is a shame.