Abstracts of Loudon Co. VA marriage records shows two Isaac Fouch marriages.
1) Isaac Fouch and Mary Steadman 8/12/1816 2)Isaac Jr. and Nancy Coxen (Alias Halling) Thomas Fouch attests brides age. 9/23/1792.
From the Fouch McDowell Bible we find that Isaac son of Isaac and Mary McDowell was born Feb 15, 1763 and died Feb 1795.He would in 1792 still be identified as Jr. as his father was still living.
He could be the Isaac who married Nancy Coxen Halling in 1792.It is possible for this to be his second and later in life marriage. The way the abstract was written it would appear that Nancy had previously married.
It's not possible that he married Mary Steadman in 1816 but it could be his son--grandson of Isaac and Mary McDowell.
This could be the second marriage or there could be another Isaac son of one of the other Fouch brothers marrying.
This probably adds to the confusion but I think there is a definite relationship.