> Mackenzie does the same type of thing in Quest with the Blaeberry Courtship. He attributes it to James Isaac Macdonald in 1919, then in 1928 attributes it to John Henderson.

I've just read what Mackenzie writes about James Isaac Macdonald in the 1919 book (should have done this earlier). Conclusion: he and John Henderson in the 1909 article are the same person. Pages 227-234, about James Isaac Macdonald, are basically a rewritten version of what Mackenzie had written about John Henderson in the 1909 article - same life story in detail, same personality as far it comes across in the texts, just a different name. In both cases, there is then a contrast drawn with Robert Langille (same name in both, although he becomes "Bob" in 1919), similar in age to Henderson/Macdonald but of "French-Swiss" origin, who can still sing ballads, although he just gets a paragraph in the article and there is much more about him in the book.

Who knows which, if either, name is the real one, but at least the singer of "The Blaeberry Courtship" is recognizably the same person in 1909, 1919, and 1928. There's no way he could also be John Thomas Matheson, however, so the changed attribution of "Young Johnson" remains hard to explain.