Mary Catherine Ferringer who married Johnstone Mercer is not Joseph and Catherine Dietrich's daughter. Their daughter Catherine married Jacob Traub/Troup and I have not seen anything to indicate for sure that Mary Agnes was married.
As for Peter and Elizabeth Miller's daughter Catherine, she is more likely to be the correct person. I did see one list of Peter and Elizabeth's children that said her name was Catherine Mary.
Did Mary Catherine and Johnstone have a son David?In the 1880 census, there is a David Mercer, 33 living in Redbank, Clarion County, Pa. Also in the household is Catherine Mercer, 54, listed as David's mother. You can look at the information on www.familysearch.org, 1880 census link.
I am researching the Ferringers and several other families from Clarion County. Willing to discuss any of them by email or message board.