Bruce, you must have the most fantastic library!
Yes, I knew many were completed from broadsides of the same song - the ones that throw me are when they say they aren't related. For instance, when Chappell set Robin Hood and Guy of Gisborne to a tune in the Dancing Master named The Chirping of the Lark (because Robin wakes up to the sound of a bird). Of course, you probably know hundreds more examples!
Since the Bodleian came on-line it's been great fun to look up the many broadsides of songs there. They are, by and large, fairly consistent - and the words are usually fairly consistent with the words from the old songbooks I have from collectors. The tunes may be something else entirely. Although I like the arrangements, I've been told the tunes in Broadwood, Barrett and Sharp are "over-arranged," which points to one of the advantages of the ABC format over midi.