The _unidentified_ Samuel Abernathy from the 1840 Ohio census could not be the same as the Samuel Abernathy found on that 1850 Brown County census because of age. However, he was most likely the father or uncle of the Samuel & the William found in 1850 in Brown County OH. The _identified_ Samuel Abernathy from the 1840 Ohio census (Clermont county) was probably the same individual as the Samuel found on the 1850 Brown County OH census living right next to William, who was likely a brother. Brown and Clermont counties do share a common border for a considerable distance and it's entirely possible that those Abernathys didn't move but that the county lines were re-drawn to make it look as if they did.
This particular Abernathy lineage seems to have had a Samuel, a William, a James, and a John in each and every generation since it's origin in PA in the early 1700's, which has complicated matters, given that brothers also had sons by the same name.
On the matter of who was the younger Samuel Abernathy's 1st wife, I presently have no concrete evidence to determine theat issue, lacking official marriage data on both Mary (surname unknown) and Casandra Purdum.
Nor did I have any knowledge of the court records concerning the heirs of Benjamin Purdum. The Illinois court records didn't mention Sarah or Isaac as heirs?
I will have to check this out. Did Benjamin leave a will, and if so, would you please email me a copy?