1. Under IATA convention, this is a QR problem as the delivering carrier.
2. Nonetheless, you are right to be proactive yourself.
3. It's unlikely that UA was able to reroute a bag initially destined for Canada to another country. My guess is that the bag is either wending its way to you through its original routing (delayed to next flight) or its sitting at IAD.
I would push QR hard to get on the horn to UA at IAD and YUL and then to their own people at YUL & DOH. Forget what the computer tells them.