Thank you for the detailed response. I figured the parents Wm and Mary were married in Mississippi, but hoped some marriage record of the kids would list parents names. It is strange that Wm is listed as R A in the 1850 census. Still appears to be the same person/family.
My gg grandmother would be the M A. McGee if I have the right family. Her name was Permelia Ann, sometimes called Melia.
Mary and Henry McGee live next door to her and her family in 1880 in Commanche county. She married Josephus Warren Albin in Comanche county, Jan 13, 1876. The Jackson you listed would not be her.
For the father, I found a W. Y McGee in the 10th Regiment, Texas Cavalry (Locke's) Company D which was recruited for in Smith county, Texas. This unit had a very high Casualty rate. If he was part of it, he very likely died in the Civil War. Too many initials used. No way to prove it that I know of.
If the Davis family you listed in 1850 and that Nancy was living with in 1860 is related, either the Davis kids with the McGees are cousins, Mary divorced a Davis (John?), or Mary's first husband was a Davis and died and the other Davis family is his relatives. (or is John Davis her brother?)