Q: When the Prakriti is emptied of the action of the gunas, or when it ceases to act, what is left in it for us to distinguish between Prakriti and Purusha?
You seem to think that action and Prakriti are the same thing and where there is no action there can be no Prakriti! Purusha and Prakriti are separate powers of the being. It is not that Purusha = quiescence and Prakriti = action, so that when all is quiescent there is no Prakriti and when all is active there is no Purusha. When all is active there is still the Purusha behind the active Nature and when all is quiescent there is still the Prakriti, but the Prakriti at rest.
Prakriti is the Force that acts. A Force may be in action or in quiescence, but when it rests, it is as much a force as when it acts. The gunas are an action of the Force, they are not the Force itself. The sea is there and the waves are there, but the waves are not the sea and when there are no waves and the sea is still, it does not stop being the sea.
– Sri Aurobindo