For the question of whether the ballad has anything to do with any real Lady Cassillis Sigrid Rieuwerts's article "The Historical Moorings of 'The Gypsy Laddie': Johnny Faa and Lady Cassillis." in Joseph Harris, ed. The Ballad and Oral Literature (Harvard UP, 1991) makes some relevant observations, including:

1) Burns wasn't the first to introduce the name Cassillis into Ramsay's version of the song, as Child seems to have thought. There is a similar version in the Mansfield Manuscript, which is some time earlier, also with the name Cassillis, so Burns was not alone.
2) Rather than the name Cassillis being introduced later into Ramsay's text, Ramsay might well have removed it from the song to avoid causing offence to living members of the family -- the present Earl was one of his subscribers, for example.
3) "Earl of Castle" in Child G could come from a mishearing of "Cassillis" (just as the same English broadside printer misunderstood the Scots "glamour" as "grandmother"), which would push the Cassillis connection back to the early 18th century;
4)The tunes for "Johnny Faa" in several 18th century books (and a group of tunes in more recent tradition, I would add from Bronson) are similar to a tune called "Ladie Cassiles Lilt" in the early-17th-century Skene manuscript, which suggests a possible association between the ballad and the family going back at least that far