I think it is possible that some of the McCartney males could have been part of the William Davidson, Jr. household. I did find a census record for Jane McCartney in Botetourt County in 1820 (201100-03010). All three daughters can be accounted for, but only 3 of her 5 sons. After the 1840 census I haven't been able to find a record of Jane.
Other than my husband's line (James), the McCartney male children have been especially difficult to trace. As far as I can tell, Mary McCartney never married, but according to the 1850 & 1860 censuses she lived with her sister Martha's family. Martha married Allen Biggs and remained in Virginia. Charlotte married James R. Tackitt and moved to Missouri.