We are not told the specifics of Margaret's death but the implication is that she died for love, i.e., a broken heart, as in all good romances and in many ballads. Don't forget one of the prime characteristics of these ballads is the leaping and lingering. Here we have a good example of leaping. As in lots of similar ballads likewise William predicts his own death on the morrow, dying of a broken heart. The whole ballad is really a brief version of some of its contemporaries.

Yes, as in other similar ballads the brown girl is his bride (Lord Thomas and Fair Eleanor) usually arranged by his parents as she has wealth which means more to the parents than beauty.

There's no need to supply any stanzas. It's all there in the ballad and indeed in other ballads. I'm not going to insult your intelligence by spelling it out.

These are Mills and Boon stories we are dealing with. They don't have to be realistic or in some cases even make sense. Many of us who love these ballads see this as part of their great charm.