So, unfortunate lesson for everybody is that anytime you are traveling on another carrier's metal, check with that carrier to be certain that you are properly ticketed and confirmed.
To the extent that this is adviseable in general, it is a 100% necessity on award tickets and a 120% necessity when CA (OP - note that AC is Air Canada) is involved. This can generally be done online, but it may take frustrating phone calls to carriers which do not staff 24-hour customer service lines or provide toll-free service (Skype is your friend).
Ultimately, as the ticketing carrier, UA would have fixed this. But, on the ground, you can bet that CA agents see many of these and when you are firm and won't budge, they know that they are inevitably stuck with you, so stand up to them and sooner or later, accept some face-saving story about how the "computer just found a seat" and walk away.
Then wash, rinse, repeat. It also pays to have a friend or colleague back home who has access to all of your information and whom you trust. Setting up a three-way call is not quite so easy from a crowded airport.