I've been able to find references in various census for the Hessons and for Hugh & Mary Ann after they were married. Their first three children were born in Madrid, New York so I know they lived there from 1858 to 1862. They then moved back to Ontario. Their next was born in Stormont. I'm sure they were married before they moved from Ontario and the land registry shows Hugh owning property in Stormont while they were in New York. I'm assuming then that the marriage was shortly before 1858 as Mary Ann would be about 19 or 20 years old.
Hugh has always been an enigma. There is no birth record, immigration record or marriage record than I can find. It's like he's one of the first in the witness protection program