Here begins confusion. Fair Margaret and Sweet William are stock characters & show in many ballads & floater verses. Child, himself, relates #74 to #73, Lord Thomas & Fair Annet. The DT version also has the whole parrot sequence. This is trad. in this song but itself a floater which I associate more with The Greycock series. I wouldn't pay much attention to names or subplots as to the core story of the ballad. (Not necessarily easy to line out either.) #4, Elf (Outlandish) Knight is a baddy, come to murder the seventh princess. She kills him instead.

In #74, Fair Margaret and Sweet William are tragic lovers, he's (usually he) a ghost and we have the night-visiting element (usually.) But the lover may be revenant or live - it may wind up a simple night-visiting song or a ghost song.

I generally "need" to place songs in relationship & sequence but there are exceptions. In this case I'd leave it alone.

DT gives a favorite version of mine as the Peggy Seeger "Greycock" and (correctly) cites Child #48. Her & MacColl's Blood and Roses series (and Sam Hinton's Wandering Folksong) do a fine job of examples of the problem.

Good luck.