I came to the Merrifield GenForum to post some Merrifield census records I came across in the process of researching my Morefield/Moorefield/Mourfield/Mofield family and just happened to see your message title.
I can identify the Mourfields of Knox Co., Loudon Co. and Roane Co, TN:they were the descendants of William and Nancy (Ashley) Morefield, who married in Surry Co, NC in 1805.The Mourfield spelling became common in this line of Morefields.H.A. of Roane County was Hardin Ashley Morefield, s/o William.He is well established in the Roane Co records--big lawsuit after his death between his 2nd wife, Martha, and his children by his first wife.Marcus Lafayette Morefield/Mourfield was a common name in this line--I know of at least 3.
As for the Thomas Marefield who married Polly Clay in Davidson Co.He was listed in the 1820 census as Thomas Morefield.He apparently died in 1827 and is named in his obituary as Thomas Morefield.I have been unable to link him definitively into the Morefield family, but several Morefield families moved into middle and west TN about 1820.Some of the families who moved into middle TN gradually became Mofield.
I'm sorry that this information doesn't move your research forward, but perhaps it will keep you from wasting your time.
Interesting that the Merrifields go back to VA and NC.The two major branches of Morefield/Moorefield can be traced back to Halifax Co, VA and Rowan, NC; the NC branch is probably descended from a brother of the 1st Halifax Co. Morefield.I do think the Morefields and Merrifields were sometimes confused in census records.