I read your message with interest. My wife is descended from Sarah Johnston and her second husband, Henry Gordon.
You quoted Charles David Satterfield. He appears to have done a lot of research with direct contact with members of the Gordon family. Do you have any information on him? When did he write his genealogy?
I have also seen information saying that Sarah was the older sister of Robert Johnston who was an aid to George Washington. I can't vouch for the accuracy, however.
There is a James Johnston who died in Atrim Township, Franklin County, PA in 1765 who had a wife Elizabeth and four sons who fought in the Rev. War, one named Robert. He arrived in PA in 1734-35. This James' will makes no mention of a daughter Sarah.
Did Nancy die at an earlier age and James re-married? Is it possible she tired of Colonial life and returned to England?