#1: Canada is not getting this car first (the build's at the factory are the same time). It's just that the Canadian distribution method is a little bit faster than the US one (I won't get into all of it but it has to do with lower Canadian volumes). Bottom-line is, the difference between US and Canada dealers getting the first shipment will only be about 2-4 weeks.
#2: US-spec Mazda3 will have more configurations than Canada (again lower Canadian volumes). Canadian-spec Mazdapseed3 is coming in 1 colour only initially (1 additional colour will be added later), and the vehicle comes in one-spec only (no factory options, just accessories).
#3: It's not worth exporting/importing any regular production car between Canada and the US - way to many hassles, costs, possibly retrofits, and so on. The Volvo (and BMW) overseas-type delivery programs are vastly different than a situation like this - the overseas-type programs is a program with Volvo/BMW USA.