As I recall, the CPS offers a number of options regarding the emergency MDC message, which are explained in the help files. Unfortunately, I'm not near my programming computers.
The basic concept, though, is the the initiating subscriber continues to send the emergency word until the sooner of (1) receipt of an acknowledgement message from the dispatch console or (2) expiration of the retry counter. I am surprised to hear that a simple key of the initiator terminates an unacknowledged, unexpired retry, since that behavior would obviate the need for a cancelation method.
So far as I am aware, there is no way to program a subscriber to send the acknowledgement; that function is built into the Stat-Alert modem on a Motorola console or achieved via a Cimarron MDC controller on a Zetron console. Again, the construct of the MDC emergency function is that it is received by dispatch, after which dispatch handles alerting and coordinating field forces.