I'm not extremely knowledgeable about your family but have done some investigation on them to sort out all the Alexanders in Henry County, TN, and to try to determine whether Mary Ann Powell was closely related to Allenson Powell. (I couldn't find the Powell connection.)
In 1850, Thomas, b about 1807/1808 in NC, was listed with wife Nancy, born about 1801/1802 in KY, along with daughters Malinda T., 14, and Sarah E., 11. There were no sons in the family; however, in 1860, Nancy and Malinda T were in the family of W. J. (J may not be correct) Alexander, 26, who appears to be the William J, who married Lucinda Faust later that year (16 August 1860). Although William J. was not in the family in 1850, he appears to be a son. Daniel P., who married Mary Ann Powell 1 April 1847, headed the only other Alexander family in the civil district with Thomas in 1850 and lived quite near his family. Other evidence that Daniel is part of Thomas's family is the Emerson connection. Daniel's probable sister Malinda T. married Ben R. Emerson 16 January 1862 (second marriage for him), and Daniel's and Mary Ann's daughter Nancy Elizabeth, born 1850, married Ben Emerson's son David B. Emerson 13 December 1866.
In 1870, female M. Alexander, 40, and J. C., 15, lived next door to D. B. and N. E. Emerson, which seems to make J. C. part of the same family.
I have it also that David B's and Nancy E's daughter Sarah Virginia Emerson married William Thomas Alexander 17 October 1888. I haven't done any research on William Thomas.
I will compare your YDNA with others in the Alexander DNA project and attempt to discover whether you match with any of the 200 plus members who have tested.