I looked at the trees you referred to at Ancestry.My guess would be that Mary Barbara was married to a Propst prior to her marriage to George Swadley.However, Pendleton county marriage records only go back to 1799.I can tell you that George did marry Barbara Penniger in Pendleton County on 21 Dec 1817, the year that Mary Barbara died....2nd(3rd?) wife.
Another possibility is that there are two different individuals.I did find a tree that had a Mary Hevener that married a John Popst in Pendleton county. She was born in 1784, died about 1860 in Barbour county.She was Mary Barbara Hevener's first cousin.Someone may have confused the two woman.Unfortunately, people sometimes get really excited when the find what they think is a match and don't follow through with the research.
I, too, am looking at Propst/Swadley connections.I am descended from Frederick Propst and Barbara Swadley.I think Barbara may have been your George's aunt. I'll let you know if I find anything.