You wrote: I think we may be dealing with different people
I don't think you are, Melissa.I have _tentatively_ identified the James Abernathy on the 1830 Champaigne County OH census as married to Elizabeth Igou, apparently a daughter of Elizabeth and Lewis Igou. They can be found in the Ancestral Files on www.familysearch.org but I caution you that the site includes little concrete source references and does not include Nancy in the family group, as clearly is the case. Still, perhaps the few clues that are available from that site would assist you.
As for James, I cannot yet even conjecture as to whose his parents might have been. But I agree with Mike that it's highly likely that he's descended from either William or Samuel, probable brothers in the Revolution, although via which son is yet to be determined.
Unlike Mike, this is _not_ my Abernathy lineage, so I'm less sure of facts here than I might wish to be. In fact, I defer to Mike for the really thorny questions on this line.