Note that Nehemiah being identified as Nehemiah Jr. doesn't necessarily mean that he was the son of Nehemiah Dowell. It just means that there was an older Nehemiah in the area. According to Find A Grave, Elijah and Jemima (Board) Dowell had a son named Nehemiah, so it may have been their son who married Elizabeth Dowell (whose ancestry isn't known). It isn't known whether Nehemiah and Mary (Board) Dowell had a son named Nehemiah. If they did, then possibly he was the one who married Elizabeth Dowell.