I'm with Steve here. As with Lloyd, much discussed on other threads, so with some of those older collectors.

There's nothing at all wrong with anyone making what they consider to be improvements in a song before publishing it or performing it. Very often, though not always, the song really is improved. And anyway, whether the modified version is better or worse will be partly a matter of taste.

What is rightly objected to is that they sometimes told lies about the songs' provenance. "Collected from Mr A B" or "learnt as a child from a nurse" or whatever, when in fact they have significantly changed whatever they initially collected, or indeed in some instances Mr A B never existed at all as far as anyone can tell.

Going back to Lamkin: it seems plausible that the earliest versions gave no motive and that someone added the unpaid mason idea, WITHOUT admitting to doing so, though without telling any explicit lies about it either. We can surmise about whether this happened and if so who did it, but we're unlikely ever to know for sure. And whether the story is better with that motive, some other motive, or no motive is a matter of taste.