Hi Brian,
Yes, I did get time to do a quick study which you mostly cover in the above. You mention the broadside 'Knight' insipid version possibly going back to the 16th century, but I see no reason why it wasn't a rerwrite by Laurence Price as claimed by him, which would make it about 1650-70. It's certainly his style.

I am on the track of a stall copy mentioned by Baring Gould and printed in Northern Ireland in the late 18th century, but I've a sneaky suspicion it will turn out to be a fairly common version of 'Captain Wedderburn's Courtship'.

I also came to the conclusion the Kentucky/Vermont/North Carolina versions were pretty much the same and well copied and probably derived from Rill Martin via Gladden.

I found the Maine version fascinating and a brilliant example of how literate translators can pass ballads back and forth between languages. Apparently Herder translated the D'Urfey version of the broadside, and then Aytoun translated the German back into English and then it turns up in Maine in oral tradition. I have similar parallels with a German ballad passing into French then Portuguese and being collected in Brazil in oral tradition, all in the space of a century.

If the Williams/Hill Martin/McQueen versions turn out to be survivals from the 15th century version without intervening print that's a remarkable, but not unique, survival.