Joe,
I am no scholar, but I have read a lot of what the scholars write and I can't imagine how Child and his collaborators in Scotland and all the scholars who have written on this since could have missed the info you post above. Therefore my own instincts tell me it is vitally important that what you say above can be verified in any way. As far as I can see they were all concentrating on the various intrigues between the Faas and later earls of Cassilis. I have most of Chambers' Journals and The Book of Days which have various ballad entries. It doesn't feature in any of the books I have by Robert Chambers.