Oh yes, the mystery author is certainly one of my great grandfather's cousins since his mother is their aunt, Sue McAfee Owens.
The main question though is who are Sue's grandparents, the parents of Mary Luvenia Cardwell, William Cardwell and his wife, Miss Ferguson? And William's brother's, Jack and George? For these four I have nothing but the names and locations mentioned in the story.
I did locate the family of James Cardwell & Sarah Crocket:
https://www.wikitree.com/wiki/Cardwell-1786. They have a William, John "Jack", George, and a Mary Luvenia! It could be that William named his daughter after his sister, and George did end up in California... And these are all pioneer families that came to Kentucky together. I located them on FamilySearch as well, but there just don't seem to be any source documents for them, and some of the puzzle pieces seem a bit out of place yet. (They seem settled in Mercer County, KY, so why would William and Miss Ferguson marry in Rhea, TN? Or are they from another branch of Cardwells?)
It seems a bit ironic that my Mary Luvenia was abandoned by her parents who I can find no trace of while her adopted Cherokee parents who raised her are easy to document...