I saw your post as I was looking for a Montreal Labadie family. (No relation to me) These are just some quick ideas that you would need to research out to see if they apply.
I think it is possible from my brief research to think that the Labadie families out of Montreal were originally part of the Arcadian migration. (Arcadians who later left Canada and settled in Louisiana became known as "Cajuns") If this is true, a study of their customs and movements over the years might answer some of your questions.
The Arcadians were a religious group who married among themselves to preserve their beliefs. If your Labadie and his wife were both living in Arcadian communities, then he was following his belief to marry within his people.
They also had very strong family ties. There was visiting of family between different settlements, and movements back and forth as family communicated about available work/land opportunities near them. A young man might move to another Arcadian community to work for an uncle or brother.
It was their custom, at least in Quebec and Louisiana, to marry young. (A webpage about a Joseph Labadie who moved back and forth several times between Montreal and Michigan states that the French in the New world were encouraged to marry young and often an 18 year old boy married a 13 year old girl.)