Thanks, Rossey. I'd bet that "Song of the North" is the missing link between the 1840 version and the song's later popularity.

The melody, like the words, is what is sung today.

Two significant facts, which you'll probably have noticed:

The publication date of 1885 comes not long before the song's sudden popularity.

Its appearance, moreover, professionally arranged and among the first few songs in a lavish publication "Dedicated by Gracious Permission to Her Majesty, the Queen," guaranteed it would have been noticed and appreciated.

And we know "Songs of the North" was influential: It also marked the   publication of Harold Boulton's "Skye Boat Song."

Only the arranger of "Loch Lomond" is given credit, and the piece is described as an "Old Scottish Song." In light of the introductory remarks on how the songs were collected and chosen, this suggests that the editors found the song in "oral tradition" pretty much as they printed it.