I have an AC ticket which includes travel from YYZ to VIE via LHR next February. The LHR-VIE segment was originally booked on BA704 (under BA flight number, not a codeshare) departing at 14:35 which gave me a comfortable transfer time of over 3 hours. However, that flight is now cancelled on Tuesdays and I got kicked to BA702 at 12:45. That gives me only 85 minutes, which is below the MCT of 90 minutes. Obviously I'll want to switch to a "legal" (and comfortable) connection.

1. There is an OS flight at 14:50, which would be suitable in terms of timing (less so in terms of service, but whatever, I'll probably sleep anyway). I assume it shouldn't be a problem to get rebooked (for free) from BA to OS, since it's an AC ticket an AC offers connections on both carriers. Is this correct?
2. Is it possible to handle rebooking in case of a schedule change via some self-service option on the web? I haven't got any e-mail informing me of a change in the first place, they simply sent me a new eticket and I had to "manually" look for what has changed, so there's no link I could follow. Logging in to MMB, it doesn't mention a schedule change either and when I tried the regular rebooking option, it threw an error.
3. If I need to speak to an agent: I can't find any chat/What's App/similar option on Air Canada's website, do they only offer support via an old-school telephone? If so, is there any EU number I can call to avoid relatively hefty fees on calls to Canada? I found some toll-free lines in certain EU states, but those won't work unless I have a phone from that state, which I don't.

Thank you