What cemetery is she buried in. Have you tried contacting them to findout more info? A family tree at Geni.com has a death date of March 4, 1904 for Mary. There are some Unitarian records for Montreal online at Ancestry and I've checked through the 1904 records but did not find her. Also, the 1901 census says they were Methodists.
I understand what you're saying, but since you don't have an actual date for her death, you have to start somewhere. Also, the cemetery record should give you a date of burial which may or may not match what's on the headstone.
There is a Pete Bowen on Geni.com that is descended from this couple and has a tree online. You should perhaps try and contact him.