Hi, Bob
Peter's text (Child C) is a typical Buchan concoction, in at least 2 ways, it is as usual more than twice as long as any other version, and the girl's age is lowered as in many of Peter's concoctions. Like you I am highly sceptical of the whole ballad as a traditional remnant.
If you need further convincing, there is some typical Buchan shite, the dreaming of his brother's death in stanza 12, some archaic language, e.g., sts 6/17, stanzas 24 and 26 rubbish, stanza 27 filched from Kitchie Boy, as is Peter's wont, and the final stanza is certainly not of tradition. Also, only in Peter's version does she having a child out of wedlock occur, likewise only in Peter's version is the nurse responsible for his murder. It has PB written all over it, like many another of his concoctions.