If you are not particular on airline I think your strategy of finding low cost options from YUL to LAX/SFO then low cost options from LAX/SFO to BKK/HKT is a pretty good one. I do this a lot as I get really bad fares sometimes from IND-BKK. I dont know how long you plan on being in Thailand etc but I would probably split it into 3 iteneraries althoug that can get confusing and a little dangerous if you have flight delays or even can run into the possibility of breaking ticketing rules but it probably would be the cheapest rout unless you can get a consolidator that works with your location. I would think there would be some good agents in YYZ but that is fairly far from your home airport......I always try to stick close to home if I can. As far as going east instead of west.....been my experience the prices are typically better going west but if I remember right YUL is more convenient for oneworld alliance members so you might be able to find some BA flights that are reasonable but still doubt it is better than going out of SFO/LAX or even YVR for that matter.

Edited as I just noticed your post said you will be in Thailand for one month. I would plan on overnighting on the outbound.