Steve Gardham wrote:

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.

Excellent points all, which I had not thought about because I hadn't deeply studied the ballad. I will add your observations to the entry (with credit to you, of course). Thank you!