An unpublished benefit is that a SE can use the AC arrivals lounge at LHR (outside of security), even if not arriving on any flight that day. Since the YVR arrivals lounge is no longer an AC lounge, I would presume that a SE can not access it, unless arriving on some AC (and other airline?) flight.

One can not use any departure lounge unless one has access to it - ie one has a boarding pass to be in the lounge/gate area (presuming the lounge is airside - which is not the case at least for the Quebec City lounge).

Once in the gate area, I would have to ask an AC insider if the unpublished benefit for SEs using the lounge while flying out on a nonAC flight (eg Westjet) still applies, or not.