I'm glad to help.
As to Simeon Cox, I have both personally inventoried the cemetery in LaGrange, and then consulted the Troup County Cemetery Book (which came out after I first inventoried the cemetery). My personal visit came in the summer of 1993, so its been a while ago. The Confederate Cemetery is called Stonewall Cemetery; its located near the old depot, and is pretty small -- I would guess no more than 100-125 burials. Virtually all are marked as knowns. It is surrounded by a high stone wall, hence the name. There is another smaller CSA burial ground in the main city cemetery, but Cox is buried in Stonewall. I cannot account for the difference in dates; I suspect that the date given on the stone is the date of burial, since you have a date of death a day earlier. But that is pure speculation. Perhaps the local SCV Camp or UDC Chapter could clear that up.
As to J. D. Cooper, there is a large Confederate Cemetery within the boundaries of Rose Hill cemetery, which is maintained by the City of Macon. I assume they also maintain the Confederate Cemetery; in fact, I'm certain of it. It is impeccably maintained, and a large Confederate National flag (I'd call it a garrison flag) flies day and night within the Coonfederate cemetery. Its floodlit at night, and can plainly be seen from I-16 just across the Ocmulgee River. It is the best-maintained Confederate Cemetery I've ever visited. I'm guessing that there are at least 600 CSA graves there. Most of these men died in the various Confederate hospitals located in Macon. I am certain that the "Old Cemetery" you memntion is the same as Rose Hill Confederate. If I gave a row number, that came from the UDC listing. I've personally inventored this cemetery, and have consulted the UDC inventory.
What I have stated about Cooper would also apply to Mock. I could certainly have mis-transcribed his middle initial.
As to Samuel W. McDonald, I have personally inventored the Confederate Cemetery in Myrtle Hill Cemetery (this is the old City Cemetery, but is still in use) in Rome. There was also a UDC cemetery survey published in the Floyd County History, published in the 1930's. Myrtle Hill is also impeccably kept up; the Confederate section is relatively small, probably 150 graves. I speculate that McDonald may have been first buried at Sand Springs, and then reinterred in the Confederate section at Myrtle Hill. There is no cemetery survey of the Floyd County cemeteries, at least of which I am aware. Myrtle Hill Confederate is a very peaceful spot; well shaded by old oaks and elms, and located almost at the top of the highest hill in the cemetery.
Georgia has done a good job of maintaining most of the Confederate cemeteries, and for the most part the great majority of the graves are marked and identified by name. However, there are a lot of unknowns in the two largest, the Confederate Cemetery at Oakland (the largest in the State), and the Confederate Cemetery at Marietta.
If I can help further, let me know.