> meeting the (mer)maid by the well or stream, some series of interactions with her...

I don't suppose the lines of asterisks that appear at this point in all three of Child's texts need any explaining, but it perhaps worth mentioning here that Child 42A is actually a bowdlerized version of what Anna Gordon sang. The original Tytler Brown MS was lost in Child's day (it turned up again in the 1930s), and all he had to go on was a copy in which anything sexually explicit had been changed. The main changes were to verse 3, which in the manuscript is actually more like its counterpart in 42B:

"O had your tongue my gay Lady,        
An' dinna deave me wi' your din,        
For I saw never a fair woman
But wi' her body I cou'd sin."

and verse 6:

He's taen her by the milk-white hand,
And likewise by the grass-green sleeve,
An' laid her down upon the green,
Nor of his Lady speer'd he leave.

Albert Friedman borrowed this verse from the Tytler Brown MS to fill in the gap in the story in 42B in "The Viking Book of Folk Ballads of the English-Speaking World". The correct text of 42A is given in full by Bronson, and more recently by Sigrid Rieuwerts in "The Ballad Repertoire of Anna Gordon, Mrs Brown of Falkland".