This discussion of prequels/sequels to ballads, or other conflations of text caused an almost instinctive discomfort which I couldn't understand at first. But the more I thought about it, I realised that it had clarified one of my own requirements for a ballad which I had never really articulated, probably because it was so deeply embedded.
My own personal preference is for a single-issue ballad where the focus is unremittingly on that issue; for example, 'The Cruel Mother' would be diluted for me if there were a lot of verses at the beginning about the love between the clerk/whoever and the woman and why it was impossible, and if it then continued with the return of the male protagonist after the 'punishment' so that he could bewail/die/be buried near her with twining roses and briars etc.... It would more and more resemble the plot of an ongoing soap opera, rather than the perfect tragedy in miniature that I believe it to be.
I have absolutely nothing against soaps, but the relentless need to keep an audience watching means that there is seldom any pause for reflection - which is what I think happens at the end of a well-sung ballad.
And by the way, I don't mean that the action of a ballad has to happen in real time. Think of 'Lamkin', which obviously takes place over a lengthy period, where the underlying impetus - consistently - is revenge. I think that's what I mean by single-issue or single-focus, but I am well aware that others like a more epic "production number" like 'Young Beichan'.
So it's just as well that there are plenty to choose from!