Great question — answering as the UphillTech team:
That sounds right. For the buffer, I would put more towards the security and US preclearance to enter again. Maybe 1.5- 2 hours to be sure. I have not been to montreal US preclearance but in Toronto it can be 2 min, or it can be 2 hours, depending on how busy and how many staff. I know one time i arrived early morning 3 hours in advance for a flight to USA in Toronto and it took almost 2.5 hours to get through a very busy US immigration. I had just enough time to get to the gate. Not sure about Montreal, but its possible its busy, that is where i would put that extra buffer time. But other then that it sounds like a good plan. I would also check when the next busses arrive incase you miss...
Full answer by adam clare: https://travel.stackexchange.com/a/194438 (CC BY-SA 4.0).
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