No experience with this, but I would keep the existing ticket and communication about possible flight cancellation with me to begin with. In case, the cancellation is starting to look confirmed, I'd try to get another flight, if that's an option. If nothing is sure, I'd just go ahead, present the "flight ticket + cancellation" as well as receipt for prepaid Airbnb in Ottawa to the immigration officer and explain the situation. If the flight indeed gets cancelled, there would be at least a few more people in somewhat similar situation and officer should not have a lot of difficulty in accepting your reasons.
Also, if a flight is getting cancelled with not enough notice time, same airline would sometimes put you on the next flight they have for this route. It does mean spending more time at the airport and perhaps spending a few more bucks, but probably better than some other options. Check with the airline or your travel agent.