Thanks, AC, and I'd love to see your verses if you're willing. I've been working on something similar using the chapbook text as a basis, and had even considered omitting the 'firing a bullet' verse because it really doesn't add anything to the story to me (and when my son first heard me sing it, his initial response was 'So he's supposed to have accidentally killed her, or what?').
I'm sure you're right about the broadside being singable to the Notley tune. I'm just not sure, reading the verses (basically in 8888 metre I think, and way more regular than the Notley version) whether that would have been the most natural tune to sing them to.
We can only speculate, of course!