how they were able to get the nose back down in order to reverse the rotation, if one of the elevators was stuck NU.
The elevator does nothing without airflow over it. As soon as the crew began an abort (reducing thrust or going into reverse thrust, speed brakes/spoilers on the wings deployed, wheel braking), the airflow/air speed over the stuck elevator drops along with the speed of the aircraft, and the down-force on the tail is removed. Down comes the nose.