All the children of James (b.1785-ish -- note I've revised) were born in SC (I suspect there were more than I have listed). The family left SC in the early 1830s for Talladega Co AL where they appear on the 1840 census.
I do have this family on the 1820 Laurens SC census, but I cannot find them on the 1810. There is a James Abercrombie family on the 1830 Laurens SC census and I suspect it is this James although the household appears to contain an additional adult female (a widowed sister or relative of James or Cynthia?) and more young males than I can account for.
The husband of James's sister Hannah (Elias Brock) died in Nov 1830, but aren't census taken in the summer of the year? Since the widow Hannah Abercrombie Brock ended up in Talladega, I will look at the 1830 more carefully. Perhaps the death date I have for Elias Brock is not correct.
I cannot be certain that all the numbers below are accurate, merely my reading of census images. I welcome any corrections.
1820 Laurens Co SC James Crumbia2-1-1-0-1-03-1-1-0-0 2 males under 10, 1 male 10-16, 1 male 16-18, 1 male 26-45, 3 females under 10, 1 female 10-16, 1 female 16 -26
1830 Laurens Co SC James Abercrombie2-2-1-0-0-11-0-1-2-0-2 2 males under 5, 2 makes 5-10, 1 male 10-15, 1 male 30-40 1 female under 5, 2 female 10-15, 2 females 15-20, 2 females 30-40
1840 Talladega Co AL James Abercrombie0-0-2-2-2-0-0-10-1-0-2-1-0-1 1 male 10-15, 2 males 15-20, 2 males 20-30, 1 male 50-60 1 female 5-10, 2 females 15-20, 1 female 20-30, 1 female 40-50
Speaking of dates: how do we know that James who left the will was born in 1740? Why not 1750? I am still shakey on his backline and his numerous siblings, but am extremely interested (& support) your theory that the Laurens SC Abercrombies arrived (in perhaps mid-1740s) in VA/MD.
I live in Texas but was not born here. My mother's main family line settled here in about 1836.