Yes, you've got the right line of descent in mind. Some of the links can be solidly proven, while other links are supported only by strong circumstantial evidence.

There is a record that states Thomas was the only child of Dougald McKeithan - so that's solid. But we have no record that states Gilbert was Thomas's son. Instead we have evidence that Gilbert was NOT Donald's son - plus we can relate Gilbert to land in SC in exactly the same area where Dougald and Thomas lived. We also do not have a record that states that Alexander was a son of Gilbert. But we do have a deed that proves Alexander sold land that had been patented by Gilbert McKeithan. Alexander sold the land prior to Gilbert's death, so Gilbert must have deeded or assigned the patent to Alexander. However, there is no record of this transfer. Moses McKeithan can be proven as a son of Gilbert. Alexander was closely associated with both Moses and Gilbert. So those two links can't be absolutely proven by records that state X was the son of Y, but the records we do have don't offer any other possibilities.

As to Donald McKeithan's relationship to Dougald McKeithan. I can't prove what that relationship was, but since Donald advertised Dougald's estate land for sale, there must have been some relationship. It also appears that Gilbert went to live in the Wilmington/Brunswick area with Donald and/or his son Alexander after the deaths of Dougald and Thomas of SC. Gilbert's first record was as a witness to a deed by Alexander McKeithan (Donald's son) in Wilmington. Soon afterward, Gilbert was in SC again where he married and lived for a number of years before moving to Bladen Co. I suspect Dougald of SC and Donald of NC were brothers, but can't prove it. Donald arrived in Cape Fear in 1739, but Dougald had been living in SC since about 1725. I have found no record of when/how Dougald arrived in the colonies, but they certainly did not leave Scotland together.