How can you stop the MED if it is in transit to the closed position?
Either by letting go of the exterior button or if using the inside, by activating the door safety pushbutton.
What is the purpose of the acknowledge button?
In the event the MED or baggage door are closed but a door switch is failed open the crew may inspect the door and press the DOOR ACK button. The CAS will change to white.
Baggage compartment access limitations?
Above 40,000 ft by crew only in the event of an emergency.