I am beginning to think that this particular ballad or family of ballads would be better classified in the way that folk tales are classified, by the mix-and-match combinations of elements, rather than as discrete items under the two Child numbers or the one Roud number.

Not counting elements that are in any of the Continental versions but absent from all the English-language ones, the elements are:
the girlfriend/wife's warning to keep clear of the (mer)maid
meeting the (mer)maid by the well or stream
some series of interactions with her, including the headache and binding of his head
return home
requests to make his bed etc
death
girlfriend sees coffin or corpse coming and asks who it is
girlfriend dies.

Child 42, Child 85, George/Johnny Collins have various combinations of these elements. They could have been created as separate self-standing stories, drawing on various combinations of these elements.

OR they could all derive from an "original" English-language version (possibly from Orkney, Shetland, mainland Scotland or Ireland). The last two might have been present at that stage or they might have been missing (as they are from Child 42) and tacked on later, being borrowed from other ballads, along with the plants-growing-from-graves element.