This is the only Abercrombie line I've worked. The lack of information about James b.abt.1785 who married Cynthia caught my attention for several reasons, one being the marriage of his son James b.abt.1805 to Lucinda Goggins, another reason being the mystery of Cynthia (a common given name in the Jowell line)and a totally unknown first wife. I've done extensive Goggins work because of the handful of Goggins and Jowell marriages. I've also done some work on Rebecca Abercrombie who married Hastings Dial as one family of their descendants settled in the Anderson/Cherokee area of Texas and were described in an old Jowell family letter as being related -- I have not yet discovered how unless it is merely the Abercrombie link.
The 1830 census may be incorrect -- I don't currently have access to the ancestry.com images so I can't recheck, though I do agree his age should be 40-50. His son James b.abt.1805 is also on the Laurens 1830 with the correct age of 20-30. James b.abt.1785 had a dau born in SC in about 1830, and James b.abt.1805 had children born in early 1840s in SC -- yet both these men are on the 1840 Talladega AL census. Thomas Goggins, father in law of James b.abt.1805, follows the exact same migration pattern: Laurens SC 1830 & Talladega AL 1840.
James who left the will could not have been born much later than 1750, certainly not as late as 1760, as he had children born in the early 1770s. I do not believe he had two sons both named James but by different wives. I'm not saying such things never happened, but such occurences would be in the catagory of hen's teeth. If there were sons who died before the 1819 will was written I gather they did not leave male heirs as surely JAC would have acknowledged such.
I would be interested in learning more about Colvill & his line, though I do not see that given name in this particular Abercrombie family. Per Allen: looks like they are linked to many Laurens families. Barbara is such an odd name for that time period, except that I read somewhere that the popular song "Barbara Allen" gained publicity at that time with the publication of sheet music of it.