I would not at all mind being sent copies of those pdf files, Otis. Especially if any of them have source citations that can be checked, which so few of them ever do. I'd also appreciate a copy of that recently discovered public document from Coosa Co., AL, so I can determine what it may contribute to the puzzle.
As for the birth date of Robert, son of John and Susanna, I'm afraid it does not at all agree with the birthdate you have for 'your' David, being a good 10 years closer to the present. Nor does the birthdate I have for David, son of William and Winifred Abernathy come close, nor the birthdates of two other David Abernathys, at least one of whom was in that general area when that tax list was made, "passing through" enroute to an area just south of the Tennessee line, where he and his family prospered.
As for several other possibles, including the less well known South Carolina lineages, there is as yet insufficient information for any kind of conclusion, since there is nothing what so ever to establish where many of them were in the time frame involved. I fear your professed determination to link up with David and Christina in spite of strong evidence against such a link may be blinding you to all the other possibilities, of which there are plenty.
I have already encountered more than one researcher who had allowed that to happen to him/her, to the grave detriment of accuracy. Oddly enough, two of those claimed (incorrectly, as it turns out) descent from David and Christina (Forney) Aberanthy. Both claims were ultimately disproven on the basis of previously little known facts that they, themselves, produced of their own free will, not ones uncovered by a more careful researcher. Yet another supposid descent from Christina's parents can be disproven because the family into which the daughter married did not appear in Lincoln County NC until nearly a generation later than claimed.
I would really rather not see something like that happen to you.
You are working in an area with scant resources, few public documents, and plenty of room for unanticipated errors. This calls for extreem care and meticulous research in order to avoid mistakes, not merely the normal level of accuracy, but a level of much greater magnitude. Which I don't yet see from you. Please do your utmost to suspend your admittedly prejudicial conclusions (based on one single item) until the evidence from public and historical documents is overwhelmingly solid and unassailable.
After all, in challenging that letter listing David and Christina's children and grandchildren and all the supporting documentation that confirms it, you (who are merely a potential descendant several generations removed) are, in effect, calling a very honorable individual and much closer blood kin (a grandson) a liar. And since it does not list a son David, as you persist in declaring there must have been, you ARE challenging the accuracy of that letter and it's writer, sir!!
By the way, you did inform me, did you not, that "your" David was aged 60 on the 1850 Coosa County AL census? I can't recall off the top of my head where you said he and his wife were born.