You're right of course; the song is older than Grayson and Whitter. After reading the DT threads and looking it up in Folksongs of North America (and not having access to Lomax's references) it would seem to be a variant of Loving Hannah, and Loving Hannah dates back a Good Long Time.
What I stand by is my assertion that every version of "Handsome Molly" was learned, at bottom, from the G&W recording. Until somebody can find a counterexample, I'll continue to believe this.
Interstingly, Lomax's Loving Hannah is "from the singing of Jean Ritchie." I'd love to learn from kytrad whether her family considered Loving Hannah and Handsome Molly as the same song or two separate songs.