"This is all the more essential in folk where someone hearing Dick's song might not even be aware of Willie Scott or the Shepherd's Song, in which case they might like to go out and enrich their lives by finding out."

Well, you're assuming that that particular song by Willie Scott is the only way Dick could conceivably have heard those words. And that there aren't other singers singing those words that Dick (or anyone else) might have picked them up from. Now you might well be right, but we're talking principles here, so it's irrelevant.

I love Willie Scott's singing too, but I think there's some confusion going on here between the fan's enthusiasm for introducing people to his recorded legacy and some sort of prescriptive moral onus on telling people where the song comes from wherever there's time.

I rather like that folkie thing of "I got this song from..." but it's optional as far as I'm concerned. Actually, if I were forced to preface every song that way, I'd be obliged to say things like "I got this song from singer X, though I think their singing is awful, and I really can't recommend listening to their version: but the words are great".