So I want to go YEG-LHR-VIE and if I do it on AC.com or via the call centre it will not allow me to connect to an earlier desired OS flight and instead forces me to wait almost 4.5 hours until the next flight to VIE.

This is because my connection time is 85 minutes and not the 90 minutes required by AC. To make things even more frustrating, BD does allow you to book exactly the same flights as code shares, with the desired 85 minute connection time without problem. Why is this?

It is doubly frustrating because as it has not ever taken me (if I subtract out time spent in the arrivals lounge) more than 60 minutes, usually 30-45, to get off the plane, exit immigration, walk to other terminal, go through security and get to the other gate to wait for the cnx flight.

Those 5 minutes really wreck my desired plans.