"Note the footnote by Percy, who printed a version with "Young man, I think ye're lyan'.""

Well, not exactly. He just says that an "ingenious friend" of his thought it ought to say that because it would be "very characteristical" (not because he had actually heard anyone singing it that way, or anything like that). He doesn't say anything to suggest that such a version actually existed.

Basically Percy's "Sir John Grehme" version looks to me like Ramsay's text gone over by someone who wanted to make the language look more Scots ("fallan", "dwellan" etc. for "falling", "dwelling" etc., "Ise die" for "I'll die", "hooly, hooly" instead of "slowly, slowly" in stanza 7, and some spelling changes in the same direction) -- perhaps it was the same person who gave Percy the "written copy" that he took it from. Otherwise the only significant differences are in the first two lines of the tavern stanza.