"Butcher Boy- Jean Robertson (Aber) 1931 Henderson"

If that's Jeannie Robertson and Hamish Henderson, the 1931 date must be wrong, as Hamish was only born in 1919. It could be 1951. If the date is right it's probably a different Jean Robertson and certainly a different Henderson.

On the main theme of this thread: a ballad maker might well blend an account of a recent murder with some of the story of an earlier one and some fictional embellishment. Thus the 1744 ballad could have been partly based on the 1656 one and the Butcher Boy ballad could have been partly based on the Wytham miller one. So not one original murderer but two or three of them.