Masato, you obviously have more information than I do. I haven't yet pulled up complete music for Miss Lucy Long. Is it available on the internet?
It is difficult to handle sheet music without the composer's name. I believe there is still a question about a couple of Foster's tunes, since first publication was under Christie. Music was printed in different localities by different publishers, often with variations, just to confuse a tyro like me.
What now makes me think that you are right about authorship is that the Blue Tail Fly is missing from Emmett's "Plantation Songs" as used by Bryant's Minstrels and published in 1860. The song had already been published in several places and apparently had some popularity by that time.