At what point are the pilots allowed to communicate with the ground crew whilst they are still in the cockpit?
no longer under their control


is a strange phrasing, the aircraft has to stop sooner or later, do you define a parked aircraft "not under the pilot's control"?

I'm saying that the ground crew does not generally connect the external "headphones" while the engine is used to power the aircraft, the connector is near the nose gear well and the cable is generally not overly long, to avoid the grond crew from being ingested by the engine, they generally wait until it is safe.