I do not live in your area nor have I had kin there, but I want to tell you I think this is a wonderful project you and and yours have taken on.
It is sad to say, but what you describe is true all over. I believe it is a Southern thing to think that we should not only respect our loved ones in life, but in death also.
I know I have kin just one county over and if I only knew where they were buried, I would mark the graves myself. It might the same for some of the descendants of these folks. It is sad to say but many times people move and loose track.
Mine were so poor back after the Civil War, that they did well to survive the day. I feel sure that for each unmarked grave, there is a story just about the same.
I know for a fact that my gg grandfather, William Robinson Vaughan's grave (who had died in the 1870's) was not marked until about 10 years ago. He had been a wealthy and well to do man and the Civil War broke him in more ways than one.
Thanks for sharing and I do wish you the best with your cemetery work.