"We also know that he sometimes lied about what he was doing and invented singers...."
I'm not sure we KNOW any such thing Pip; we SUSPECT that he might have done so, but we have no evidence that he did and it seems a little 'rush to judgement' to be so catergorical.
If someone had presented me with a list of Traveller John Reilly's repertoire (Maid and the Palmer - et al) or Duncan Williamson's magnificent ballads and his enormous collection of traditional stories, thirty odd years ago I might well have echoed your sentiments about these. Now I have come to the conclusion that we know so little of the living tradition that we only KNOW an iceberg's tip of it's quality, content, function, mechanics..... anything really.
In the light of our own ignorance it seems more than a little unfair to be so definite that he 'lied' about anything.
The same accusation was constantly being made about MacColl's father's songs - I confess I was one of the cynics until a contemporary of his, Eddie Frow, told me "William had hundreds of queer old Scots songs and bits of songs"
One thing I do know for certain; the fact that a song has never been "found anywhere else in the traditional singers repertoires" does not mean that it wasn't there - want a list of one-off songs and ballads that have turned up in Ireland over the last half century or so?
Jim Carroll