Joe
'I wonder if the ending of this song in the Percy text might not be taken from Child 20'

I think the RBW who made this statement is Bob? It's actually much more likely the other way round. The earliest version of Child 20 has none of the pennances, and in fact they only occur in a small number of Scottish versions. My own take on this is that some enterprising Scots antiquarian added the verses to a version of The Cruel Mother having taken them from Percy's version of Child 21. Once this has happened it takes but a few years for the new version to enter oral tradition.

Apologies if this is repeating what I've said earlier in the thread somewhere.