i'm surprised all those people don't know the fact that a US citizen does not need a PR card to travel to canada by air. Perhaps it's how you are phrasing your question because you can cerrtainly board a plane to canada on the strength of your US passport, and then use your COPR to get through immigration once in canada. So you will want to bring your COPR (landing document) with you so it will make it easy for CBSA to confirm your PR status upon entry back in canada.
http://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=1116&top=16
i would recommend calling air canada again, and ask if a us citizen needs anything but their us passport to fly from mexico to canada and see if they give you another answer. you are not obligated to tell them you are a canadian pr, so leave it out of your conversation. just use your us passport and don't tell the airline about your canadian pr status.
also, i would recommend calling the US embassy, not the MP. Obviously MP offices are not knowledgeable of US citizen's rights to fly in and out of canada with only their passport. As a US passport holder, your pr status in canada is not relavant to your travels.