I spent a little time looking at John B Powers b. 1811. I do believe he is the brother of Rebecca , Nancy and the others. His wife Nancy, was born close to the time of the your Nancy A Powers(b. 1815 or so). At first I thought they were the same person since John's daughter was also named Nancy A Powers. But when you bring Nancy A's daughter, Sarah, into the mix it shows that they were not. Sarah's birth is pretty much established as 1859 and she is not shown living with John B on the 1860 census. If her birth were 2 years later, I would suggest that your Nancy A and John B's wife were the same person.
It appears that John B. Powers wife , Nancy, died between 1860 and 1870 when he is shown in Fayette Co with only his daughter. I believe George M D Powers listed his parents as John and Ann or Amy because he probably lived with them until he was old enough to work outside the home. John B may have been the male role model he remembers. John B's wife Nancy may very well be his second wife, the first wife being the mother of Joseph Powers.
There is a very large age difference between Joseph and John B's daughter (b. 1857)
which suggest to me that there were 2 wives. Also, John B was living alone in 1850.
I do think George M D is a Bennett.
In the 1850 Smyth Co Census, how old was the Nancy Bennett in Sally Bennett's household? Was she old enough to be George MD's mother?
There are also some gaps for Rachel and Nancy A Powers. Where was Nancy in 1870? Where was Rachel in 1860 and 1870?
I can't speak to Campbell Co. as I don't have any info from there. However, there are some researchers attributing a Nancy E. Powers who married a Duckworth to be the daughter of John B. Powers. Her death record shows that she was a daughter of an M. Powers (probably Michael). What happened to all of these people?