This sounds like a big risk where the consequence is OP may have to pay a walk-up fare or rent a car and drive YUL-LGA.

YUL-LGA is operated by AC not UA. If flight is missed, OP could end up in a situation where no airline takes responsibility because connection is inside of MCT. Maybe UA can force open a seat on YUL-EWR operated by UX.

Also note that if the arrival of inbound is in the afternoon, passenger might be stuck in the corridor leading to the arrival hall as airport staff is controlling passenger flow by roping off access to the elevator/escalator. While airport staff have been letting NEXUS members thru, these passengers will have to "work their way through the crowd" to get to the airport staff.