Which dates? I find SN and AC prices are normally pretty much the same.
Tango Plus (which V or W belong in) give you full miles.
As to flight numbers, yes it appears that SN gives you two number, one for each leg. On the way to BRU they may let you stay on board. On the way back you'll need to go through Canadian customs and immigration at YUL, which will entail picking up checked luggage, and returning it after customs.
Not sure why Expedia would be cheaper. I don't do business with them. BTW I would look at matrix.ita.
If you have not got the reward yet, i would also look at going through FRA, and check on LH.