Thanks; context is vital in questions of this sort.

Having said that, there isn't much I can add at the moment. Broadside and oral versions of the song are many and widespread, but I don't think I've ever come across one where the hero is anything but a butcher by trade. Perhaps, as you suggest, elements of another song have crept in in this case; I suppose at a pinch that the singer (or Tugwell) may have had Paul Jones in mind when giving Johnson a first name and a new occupation, but that may be quite wrong.

As to the epilogue, it may perhaps be a distorted memory of what appears to be the earliest known print form of the song: a broadside called 'The Three Worthy Butchers of the North' published by Phillip Brooksby at West Smithfield, London, in the last quarter of the 17th century. It was written by Paul Burges, and in the final part the surviving robbers escape justice "for they took ship at Yarmouth, and so went over sea". See Roxburghe Ballads, VII, 59-63. The Burges text is also quoted in Helen Hartness Flanders et al., New Green Mountain Songster, 238-244, along with a set from Vermont tradition which, unusually (most oral versions derive from later broadside re-writes in which the action is condensed and the original ending dropped) derives from the earlier form; there, the robbers "took shipping for Portsmouth And sailed far o'er the Main".

That leaves your question mostly unanswered, but at least provides some speculation as to how the unusual elements mentioned might have found their way into that particular Devon version; though not where those elements might have come from. Perhaps something will turn up later.