Hi Richie,
I've incorporated "Polly's Love" in the comparative table, as you suggested, and e-mailed it to you and to Steve.

As you say, most of the British versions seem to come from the Polly's Love broadside. William Christie doesn't say whether what the singer sang was the same as Buchan's version, but he does make it clear that it was an unusually long song, which makes it sound more like the old "Gosport Tragedy" than the 11-stanza "Polly's Love". Also in the North-east of Scotland, I see that "The Gosport Tragedy" is listed in the contents of volume 2 of the Greig-Duncan Folk Song Collection, but I don't have access to the book to see what version of the song and how much of it is there. (Ethel Findlater's version seems to be close to the Deming text, and I guess it could have come from an American source -- I'll try to transcribe it when I have time.)

I don't really see why George Dunn's version would support an earlier version. As far as I can see it all comes from "Polly's Love", apart from the first two lines about "two brace of kisses", which don't obviously belong with the rest, and could easily have been added anytime in the 150-odd years between the printing of the broadside and the time the recording was made.