If going to Ottawa, my suggestion would be to avoid booking domestic connecting flight to Ottawa from either Montreal or Toronto. If you do, make sure you're not over-paying for that leg - AND most importantly that the airline will put you on next available if you can't make your first connecting flight. (Should probably work if air canada booked all the way through, don't know about others).
In both cases, there are land options that are fine (train or from montreal you used to even be able to get hire-cars eg 'limousines' not that expensive) or can book a last-minute flight and take that if cheap enough. Ottawa's only ~2 hours from Montreal by road.
Mind, this might mean having to stay overnight in a motel or hotel. And I'd undrstand some might want to pay to just get it over with.
Again, main thing to avoid getting stuck with a connecting flight you can't possibly make because immigration took eg 2.5 hours instead of 1.5.