I am surprised that people keep pushing this back and forth between AA and UA. AA sold the OP a ticket from YYZ to SFO via LAX. How AA provides that service is up to AA. Since they did not provide that service in a timely manner, I would direct all correspondence to AA. Let AA make the OP whole. Even if it is UA's fault, let AA collect from UA. Why should the OP be getting in the middle of what is essentially a transaction between AA and UA?