They don't need to follow you around a Canadian airport, as they have the full manifest long before you land. All they need to do is put names to faces.

Going through Mexico is one way to get to Cuba, but Costa Rica, El Salvador, any country is good. Just make sure the flights are separate, as far as laying over a day, that's not important. Fly American or whatever round trip to El Salvador, then TACA round trip from there to Havana. Don't use credit cards while there and your chances of getting "caught" are slim to none.