Your e-ticket/receipt should state that AC flight XXXX is operated by United. It's probably on there somewhere. You also always need to check in with the carrier operating your first flight.

You don't have access to priority security as a Nexus member when traveling to the US. Why, I don't know, but the Nexus priority security never applies there. Did you see a sign somewhere? In fact, I would say your pretty lucky you only had the lady "protesting," IMO, ignoring security personal/instructions, and "barging though" a line as you describe can likely land you in pretty hot water - at minimum, they can make your time through security pretty difficult if they want, and you probably don't want to know about worst case.