I have been trying without success to obtain a look at the 1890s. Child canon. All I have is the CBDB which gives the title as William an Lady Maisrie, this I understood was a supernatural ballad unlike William and Lady Marjory which if nothing else relates to flesh and blood as shown in the verse:--
"It"s noo this couple tae bed are boun
Tae tak there pleasures there o
And it"s lang ere lang aa the play was ouwr
And Willie he slept soun o.

Robertson and Murray are not to be equated with your book learned collectors but as herds they were meeting folk of their own class who, though literate, were well versed in the oral traditions and songs of their area, where, in those days of limited travel, they were probably as much thirled to the area in which they were dwelling as were their flocks. Bearing this in mind and it being only 30 years or so earlier that the great harvest had been made who can say that the published version was known to their informant.

Thomas Murrays book of poems ran to some 7 editions.

Hello Mike---it must be ten years now since we last met.