A lineal descent can easily be traced between members of the Elfin Knight family albeit some of the links are quite scarce. I suppose it could be argued that 'Acre of Land' is a separate song because it only utilises half the original and has changed the purpose of the song completely. But with the 'St James Infirmary the odd isolated bits only make up a small part of the new song, almost commonplaces or motifs.

I accept that 'St James' is a new creation, but not that it's just a 'very distant relative'. If Tom Sherman is part of the lineage then we can add 'Old Joe's barroom' to the list of common features that Pamela suggested above and to which I added the six plus six mourners. Apologies to Lighter, however, for dismissing 'Rake and a Rambling Boy', which in it's British antecedents does of course have similar funeral arrangements.

As for Child 2, you could argue just as well that the impossible tasks are 'commonplaces' (the riddles in Child 1 certainly are). Is a battle for a mortal's soul with a supernatural being an identical scenario to a bit of idle banter between lovers, mediated by a third party on the way to some fair or other?

We have done this with the even more difficult and ancient 'Died for Love' family, i.e., given them separate numbers

Well, good luck untangling one!