Where did they live in the US? The marriage records for some states will list parents. Also, the marriage records for their children may list Mary's name. The death certificate is a good idea as well.
Also, have you looked for an obit for John or Mary?
I see a WWI registration for Frank McMillan Aiken- if that is the right one, his middle name may be a clue.https://familysearch.org/pal:/MM9.1.1/K6JR-41H