Here's what ended up happening.

A fabulous supervisor from Travelocity.ca called me back and picked up where the original agent fell short. He managed to get AC to refund the ticket. It took ages with me spent on hold, but it was worth it! When he initially requested the refund, AC balked. But with some pressing they relented. (I guess the fact that I was right at the limit of the '2 hour' rule helped, but I don't really know how it came together exactly.)

It won't end up costing AC anything, as I've rebooked my itinerary with them (directly instead of through a "booking agent"). The benefit of getting the refund and then rebooking is that I've shaved 2 days off the end of my trip which it turns out I don't need. (It would have been hard to do that through Travelocity without facing some penalty.)

I'd still like to see the '2 hour' rule in writing somewhere, if anyone knows where to find it.

Thanks again for all the help!