It would be in the range of no less than 5 hours longer to back track to YUL or YYZ vs a direct. 6 or 7 longer if you are concerned about weather and want to make that "central" Canadian CX.
A stopover in YYT couldn't be 2 hours on the ground. And your inbound A/C is for sure already there, because you are on it. Obviously, other things could go wrong.
I'd always want to fly the right direction, even doing a touch and go on the rock.