1. Yes they can.
2. How about a nice detox dinner at a decent restaurant in the northern part of the city (to be recommended by FT members in YYC)
3. Certainly. Just make sure you leave yourself a bit of time to get through security and prearrange a cab back to the airport, but I don't foresee any issues if you return to the airport by 23:45 or so.
4. You'll clear customs at your first Canadian stop: Calgary.
Have fun.