The SC land that Donald McKeithan was advertising for sale in 1747 actually WAS Dougald McKeithan's land. I was able to match the tracts in the ad up with Dougald's plat records. To make it more interesting, Donald's son also named Dougald was in possession of some of that land in 1750 when he wrote his will. Looks like James Aiken bought some of the land before Donald even advertised it - and then later one of the tracts that Dougald d 1750 held too.
It seems very probable that Donald McKeithan 1739 immigrant to Cape Fear and Dougald McKeithan of SC were brothers, but we still have no absolute proof of that.
I believe I've also presented records that indicate the only possible father for Gilbert McKeithan was Thomas McKeithan. Thomas's widow Deborah Thomson could not have been Gilbert's mother though. Thomas must have been married previously.
I also believe Gilbert McKeithan went to live with Donald's son Alexander - or maybe with Donald - after Thomas and Dougald died, then later returned to the remaining McKeithan land in SC as an adult.
The records are all on the page - with scans of some of the originals. A second page gives land records for the Black River and Black Mingo Creek area. It's involved, but if you spend some time studying it, you can understand the neighborhood and how it evolved.
I put a lot of hard work into this page. I hope it was worth it! Would love to know your thoughts/opinions even if you disagree.