Brian wrote
many others were racking their brains for half-remembered fragments from their youth, at the behest of a collector.

I would imagine that was the norm after a singer has established a relationship with a song collector. They wanted to please them. I can think of several examples from the old singers that I have talked to. In an interview with Johnny Doughty after he had been visited by Mike Yates a couple of times, he talked about sitting in his net shed trying to bring back more of a half-remembered song to record for Mike - often with some success. Of course, he also "remembered" a good old sea song after he had heard it on a Spinners' LP but by the time he got it the way he wanted it, it fitted perfectly into his repertoire.
George Belton and his wife often came to our club in Lewes and sometimes I would catch his eyes lighting up when a song was going on, followed by an intense whispered conversation with wife. Sure enough, a couple of weeks later he would be back with his own remembered/reconstructed version. I also remember him chuckling his way through Sydney Carter singing his composition Mixed Up Old Man. He must have got the words from somewhere and that became part of his repertoire sung to a tune that was closer of Villikins & his Dinah than the original.

I was at the funeral and then a very well attended Memorial event for a very popular local singer in November; over 100 people there. One of her daughters came up to me and said, "Vic, can you sing Binnorie? It was one of mum's favourites." I asked her to give me time to think about it and I would. I had never learned the song but when you have been around folk songs and ballads for 60 years, you actually know far more songs than you realise. I sang the song and managed to get through a version that I realised afterwards was partly from Lucy Stewart and and partly from her neice Elizabeth.
It would be easy for me to imagine a source singer being able to produce a song for a collector in that way.
Some of the songs that Caroline Hughes recorded for Peter Kennedy sound like a combination of floating verses and some that she made up on the spot, but Peter was paying her 50p per song, so she wasn't going to say that she didn't know any more, was she?