I'm connected to the Mercer family and may be of a some assistance.
I have Martha M. Mercer married to James Snell around 1864. As well I have 5 children mentioned Ella; Enoch James; Barbara; Mary; and Polly. Also have her obit.
Martha's parents were Robert and Mary (nee Anderson) Mercer from Adjala Twp. Simcoe Co. They are originally from Ireland.
I also have info on Martha's siblings if you're interested.