I recommend you check with the airline to find out if customs clearance is required for all passengers whether or not they are in transit. The last time I arrived into Canada on a flight from the USA, there was no "transit" option, but that was in Calgary. If you do need to clear customs, wou would have to provide proof that you had a seat on that flight to Europe, in which case you should be allowed to proceed because that's no different than the folks who are crossing the border in their vehicle on the way to Alaska. They must provide proof that they really are going to Alaska and when they do, they are allowed to proceed.