Hi Ross! Have you ever seen the book "Scott County, TN and it's Families"? This book was published in 1988. You can get more info from the Scott County Historical Society. The address is: PO Box 7 Huntsville, TN 37756
There is info on John (the sheriff) on page 119. On the same page there is also info on his parents, siblings and the different spellings of the name. In fact, John is listed as Llewellyn.
My brother and I are descended from Anderson Grant Lewallen through his son Andrew L. Lewallen who was one of Sheriff John's brothers. So, I guess that makes us umteenth cousins!!!
I would be happy to share what I have. Please let me know if I can help.