"...the young ploughboy being pressed into the army and leaving swearing he'll be back and marry the girl and the girl says basically that she hates him..."

More generally the song has him as having volunteered - in which case, whatever he says about marrying her when (if) he comes back, it's pretty likely the girl would say she hated him. Who'd blame her?

True enough, you get different variants filling in bits of a story in different ways, and then combining again in ways that mean there are inconsistences. So someone amends it to sort them out, and the story changes again.

Songs have an existence rather like atomic particles are supposed - there's a sort of cloud of possibilities, and you can't pin them down exactly.