Jon,
No. It's almost impossible to say what was and what wasn't faked, so they are simply untrustworthy. Christie reproduced quite a bit of Buchan's texts which are very suspect anyway. My own attitude to this is if the editor is a known fabricator I simply don't trust anything they published. In the same way as I wouldn't accept anything in say Sharp's publications. However one can at least in Sharp's case go back to the manuscripts and rely on this. As far as I know, as with Buchan, there are no manuscripts/field notes for Christie.

At the same time, let us remember that interference with folk material was almost the norm at the time. It had to be tidied up for sale to the middle classes and some editors simply had more imagination than others.

As for Bronson, like Child, he wanted to be as inclusive as possible, and wasn't unduly concerned by reputations.