Gibb Sahib wrote:

I think we can treat [Le Messurier's authorship] with a pretty high level of confidence.

Given that it is attested not only by the folk song books but also by the independent Dictionary of Newfoundland and Labrador Biography, I think this a safe statement.

It's less clear as to whether LeMussurier's composition had "caught on" much in oral tradition or if, indeed, due to the acquaintance Murphy was able to give the song a platform in his book.

I would put it this way: around, say, 1960, almost every Newfoundlander knew it. The Doyle songbooks were everywhere. I don't index pop-folk songbooks, but I have a lot of them from Newfoundland, and it's in all of them. It's in beer company songbooks, and tourism booklets. It's pretty inescapable. How it came to be so popular is a different question -- it's noteworthy that it's not in Leach or Peacock, for instance, though it is in Greenleaf/Mansfield. It's almost a "national" rather than a "folk" song. But that's a distinction much less drawn in Newfoundland than elsewhere. Newfoundlanders actually sing their anthem "Ode to Newfoundland," e.g., and not just at sporting events. :-)

Jigging and jiggers, FWIW, were not a Newfoundland-specific technique. But they are particularly associated with Newfoundland because... cod. No place else in the world was so dependent on cod as Newfoundland, and cod was caught with jiggers, and so it became a word that every Newfoundlander knew, whereas it was a specialized term elsewhere.

I would take minor exception with one statement: LeMussurier published a public notice (government business) in the advertisement section of Murphy's book itself, and it would seem likely that the two were personal acquaintances.

You probably haven't studied as many old Newfoundland books as I have. Almost all of them have those big advertising sections, and I'm pretty sure they weren't all based on personal acquaintance. The printers often brought in the ads. I'd call it possible that they knew each other, but the evidence isn't strong enough to justify the word likely. This is of course a nitpicky distinction.