It sounds like you are making progress. I have never been able to pull up Nancy's 1860 census record for it always directs me to Scott Co., VA where another Nancy Powers lived who was of a similar age. It is interesting to note how close in proximity the Powers were living in 1860 except for Rachel. John B was house 773, Rebecca had been next door in 1850, not sure about 1860, Mary Caroline was house 779 and Nancy was 816.
I didn't realize that George was shown in both households of Mary C and Nancy's in 1860. I wonder if Nancy was also shown in 2 households in 1860? The Nancy A in John B Powers house was of similar age, and might just have been visiting. John B's wife may have died earlier leaving him with a baby to raise, perhaps that is also why his son, Joseph and family were living there. What stories we can imagine with just a few facts!
Good luck with the DNA search. This may be the most reliable path to follow.