Hi Thomas! Welcome to WikiTree! Thank you for your question and for linking the profile to your question!
I hope you don't mind if i suggest a few things - maybe you have already tried them, but i thought i'd mention them just in case.
First of all, I really feel for you regarding the dozens of Isaac Odells! I don't think there is any shortcut for trying to follow the careers of each to make sure you can rule them out as yours!
Would i be right in suspecting that you are descended from most or all the families in that part of the Shenandoah Valley in that time period? If not, you might be able to narrow down who the MacKay father was by matches to his mother's line. But if everyone is related to everyone else, it might be harder. Have you read Roberta Estes' accounts of sorting out a similar sort of line in her family - but not with an adoption thrown into the mix!
Maybe someone else will have a smarter and quicker answer, but my suggestion is to just keep following the trail of the available information, and let it grow into a picture, without particularly setting out to prove anything. The funny thing is that you can never predict where the info might be -- in a will, in a land transfer, in a marriage registration document -- or something else altogether!
I'm interested when you say that your Isaac is a Y-DNA descendant of Robert MacKay. Is there anything to preclude your Isaac being a descendant of anyone else in that Y-DNA line--a brother or uncle of Robert, for instance?
Do you have reports on Y-DNA samples from other men who bear the Odell name from the area but not in your paper trail line? In other words, do you know for sure that there is distinctly different Odell Y-DNA that Isaac would have had if he was biologically an Odell? it would be interesting to know how far back this adoption might have happened. You might have to be a very painstaking sleuth to get this figured out.
Best of luck on it!