Paul,
Yes the 2/3 stanzas are regarded as a commonplace but it would still make an interesting study trying to guess which ballad had it first and so on. Harry may well have been right about it migrating from Lord Lovel to Barbara Allen, but I'm pretty certain equally it didn't start life with Lord Lovel.

Just as a matter of interest, does anyone nowadays sing Lord Lovel as anything but a joke? Many of the Child ballads were burlesqued/parodied in the 18th 19th centuries and in some cases only the burlesques seem to have survived into the 20th.

I was going to state 'thread drift' at this point, but then it's all about making me laugh! Or is it?