Once you have a few reps of establishing grip and extending, take a water bottle and strike the front of the muzzle and miss it every so often just to emulate some recoil so they can feel the shot. You can even have them pull the trigger a few times so they basically are apathetic to the boom and the impulse, whereas when you miss and don’t go too far towards your face. Because that’s a noxious stimuli, they will be building a rationalized apathy so that tight, locked in grip will be doing all its work without any proactiveness from the shooter. This is critical because any change in muscular contraction can ultimately manifest into a dreaded flinch.
Therefore, for final grip establishment, have them pull the support handle away just a little bit from the pistol and reestablish the support hand near the trigger guide and roll out to that final grip.