Exciting that the land description matched up.So it seems that William Houston was most likely Mary's husband.He likely died between 1839 and 1845.
Until the 1880s, in most parts of the US, women could not own land in their own name.They most generally were limited to a total value of $150 in goods when their husband's died.Robert Houston was most likely a relative of William's, possibly a brother, who could "hold" the land for Mary's family.Have you tried to research Robert Houston?Have you checked for any probate records in the 1839-1845 time period?I would check Itawamba, Tishomingo, & Marshall counties, and any counties that they previously were in that early time period.Most early records were lost in the courthouse burnings during the civil war, but you might get lucky.
Do you have an Ancestry.com membership?If not, you can access and print the original Land purchase of William Houston at the General Land Office records on the Bureau of Land Management's website.
As to the Native American side - virtually every family that hails from Tennessee, North Carolina, and parts of Mississippi and Georgia, has stories of native american ancestry.Unfortunately, probably 90% of them are not true. Unless a family member registered as Native American, or was shown as such on censuses, it is very hard to substantiate.These stories seem to crop up in many families in the late 1800s or early 1900s as our fear of "Indians" decreased, and fascination with them increased.So the stories go back way before the memories of any living family members.
There was an incredibly large Scots-Irish population early on in Tennessee, so my inclination, with the Sullivan name, that Scots-Irish heritage is most likely. Some Scots-Irish did marry native americans, so it is possible that Mary was at least part native american, but again, hard to substantiate.