Hi,

A different form of refrain with the "true lover o' mine" ending is found in Peter Christie's Scottish version (James Madison Carpenter Collection, JMC/1/8/1/C, pp. 11583-11584) that was learned from his mother and surely dates back to the 1800s. In this form, the Elfin Knight asks three questions and the maid asks three questions in return. One of the early texts not mentioned by Child is from The Scots Magazine, Volume 69, dated 1807 and this no doubt dates back to the 1700s. The author, Ignotus, "had it from a person who heard it repeated to him when a youth by his grandfather; who also was acquainted with it in his early years." The text and his comments are here:
https://books.google.com/books?id=ceE5AQAAMAAJ&pg=PA527&dq=%22As+I+gaed+up+to+yonder+hill%22&hl=en&sa=X&ved=0ahUKEwif7v7GvsLZAhV

[He:]
As I gaed up to yonder hill,
(Saffron, sage, rue, myrrh, and thyme,)
I met my mistress her name it was Nell,
" And lass gin ye be a true lover o' mine.

"Ye'll mak' to me a camric sark,
"(Saffron, sage, rue, myrrh, and thyme,)
"Without either seam or needlewark,
"And that an' ye be a true lover o' mine.

"Ye'll wash it out at yonder well,
"(Saffron, sage, rue, myrrh, and thyme,)
"Whar water ne'er ran, nor rain ne'er fall,
"And that an' ye be a true lover o' mine.

[She:]
"Now, Sir, since you speir't me questions three,
(Saffron, sage, rue, myrrh, and thyme,)
"I hope you will answer as mony for me,
"And that an' ye be a true lover o' mine.

"Ye'll plough to me an acre o' land.
"(Saffron, sage, rue, myrrh, and thyme,)
"Atwixt the sea beet, and the sea sand,
"And that an' ye be a true lover o' mine.

"Ye'll till it a' wi' yon cocklehorn,
"(Saffron, sage, rue, myrrh, and thyme,)
"And sow it all o'er wi' a handfu' o' corn,
"And that an' ye be a true lover o' mine.

"Ye'll cut it a' down wi' a dacker o' leather,
"(Saffron, sage, rue, myrrh, and thyme,)
"And lead it a' in on a peacock's feather,
"And that an' ye be a true lover o' mine.

"Ye'll thrash it a' wi' a cobbler's awl,
"(Saffron, sage, rue, myrrh, and thyme,)
"And put it a' up in a mouse's hole,
"And that an' ye be a true lover o' mine.

"And, Sir, when ye hae' done your work,
"(Saffron, sage, rue, myrrh, and thyme,)
"Come to me and get your camric sark
"And syne ye shall be a true lover o' mine."

* * * *