AA rules require pax to be at the departure gate no later than T-30 from international (including Canada) departures. While AA won't generally offload you at T-30, if the flight is overbooked and at risk of oversale, AA will do so to prevent IDB/VDB.

As others note, there is no way to prevent entering Canada and then going through US pre-clearance. CBSA will be expedited, but nonetheless a process. No such thing as "transit" as might exist in some countries.