Have you searched for the marraige between Mary and Mr. Propst? Since she was a 23 year-old widow, she couldn't have married him more than 10 years befor 1887. Also, you may find a search for Propst deaths and probates in the years preceeding 1887 useful. Of course, you should search for her death in 1898-1900 also. Sometimes the death record has information about the parents, although it is often not trustworthy.
I searched the 1880 Census at http://www.familysearch.org
for Propst in Pendleton Co. VA, but there are LOTS of them! Is Mary with Ephraim in the 1900 Census? That might help you pin down the time of her death more closely.