I think that you have more information than you may think. You know that members of both Frank and Mildred's families lived in London, Ontario. You know the name of Mildred's daughter and who she married. The London Public Library will probably be able to help you with obituaries. If both Frank and Mildred were living in the London area, it is entirely possible they were in touch with each other. You have stated that Doris was contacted to inform her that one of her cousins (Frank's son) had died. Obviously the person who contacted her had a connection to both families.
I noticed something interesting on the 1911 census form through automated genealogy. The enumerator for the district was Malcolm McNeil. He could be the same person as head of the family where Mildred was a domestic servant. If Mildred married this Malcolm, probably after the death of his wife, he was 28 years older than her. He had sons older than her and his youngest son was a student teacher.
I also couldn't find anything on Frank Albon/Allen on either the 1901 or 1911 census so I would be interested in where you obtained census info. I did find a Frank Allen, right age, 1911, living in Winnipeg.
I think you need to organize the information that you have with all the dates, places and names. If this was my family I would definitely concentrate on London, ON.