"That small changes that make the song work better for listeners tend to be kept? That verses that don't make sense because they have been mis-heard or include words that have fallen out of use are dropped or changed?"

That's what Sharp thought, with his 'pebble smoothed by the waves of the sea'. The problem is to demonstrate that there actually was a steady 'improvement' in the songs as they were passed on down generations. I think there is evidence that some of the awkward edges of broadside lyrics were smoothed away in oral tradition, but on the other hand lots of songs were collected that were garbled and contained nonsensical and/ or misheard words.