Whilst applauding your very ambitious attempt at comparing all versions I can't help thinking there is very little evidence for most of your observations so far. The various rewritings, additions and omissions will very rarely throw up anything conclusive. Any one of a number of versions could be the original which existed in the same form earlier and has since disappeared. The idea of an earlier ur-text is therefore pure conjecture. However, if you do come up with something even likely it will be very useful.

I don't think you've yet considered the burlesque texts. Some of the forced rhymes in some stanzas smack of burlesque and it's quite possible even some of the earlier texts were burlesques or parodies. Certainly in later centuries songs passed back and forth between burlesque and serious song. William Taylor, Lord Lovel, George Collins, The Drowned Lover etc.