Weird, I just wrote this, clicked on 'submit' and it vanished. Here I go again.
On the subject of tune fitting I think that the old singers had a more flexible approach than the modern singer. Hamish Henderson recalled mentioning a song to Jeannie Robertson and how it resembled her 'Gypsy Laddie'. Next time they met she sang him the ballad set to the tune of the other song. Similarly, I found a Flemish tune called, 'Schoon Lief' (beautiful girl) which fitted 'Barbara Allen' so beautifully that it is difficult for me to believe that the two don't belong together. (Liz and I recorded this on our second album)
The first ballad I knew was 'Raggle Taggle Gypsies' from my Dad who couldnt hold a tune very well.Then I got 'Lord Randal' from my Mum, who could. I then learned 'Barbara Allen' at school. I didn't like any of them particularly as a kid and saw no real merit in them until much later in life. I wonder if ballads are a bit like food and drink? Perhaps you need to be older and have a certain amount of life experience under you belt to fully appreciate their sublety?