I have been in contact with a descendant of Robert (b. 1842/3 AR), son of Elizabeth.His information is that Elizabeth's surname was Cayce and her husband's given name was John (b. 1798 KY), son of Matthew Adams (b.1768 VA) and Esther Barnett (b. Abt 1770).References were not provided.In a private message I will send you my notes on this family and the contact's name and e-mail address.
I have no comments on your conjecture about Benjamin except that I would agree that he might have some connection with Elizabeth or her missing husband.Benjamin is a German name so it is also possible there is a Wolf family connection.
The most promising theory I have seen about the siblings of James, husband of Agnes Wilson,is they were William (b. 1717/23 IRE, of Mercer co., KY), John (b. ca 1830), and George b. 25 Dec 1833 IRE).While there is some property and tax association with a James in Botetort Co., VA, and Madison/Garrard Co., KY, there is no direct evidence of a brother connection.There is also no association after James and Agnes moved to Christian Co., KY, about 1799, and thence, to Hopkins Co., KY, about 1809.I would be very interested in any info you may have on possible brothers or the parents of James.