Terry: "It is curious that some of the loveliest melodies were those to which the lewdest kind of words were usually fitted."

That certainly sounds like ad-libbing. The "loveliest melodies" are usually said to include, in roughly descending order) "The Wild Missouri" ("Shenandoah," even though Shenandoah doesn't always get mentioned), "Away, Rio!" "Santy Anna," "Blow the Man Down," and "A-Roving."

Long ago I mentioned that one embarrassed American collector had called an otherwise ordinary version of "Matty Groves" a "bawdy song" just because the singer had a line like "Matty screwed Lord Donald's wife." He apologized for printing the song, but it was a Child ballad, after all.