@Rushed approach: "It's interesting that with the commotion in the cabin, terrain warnings and then the GPWS going off and with the ground rushing towards him that he managed to breath normally whilst supposedly fully conscious despite supposedly having mental health issues."
It is possible that the copilot might have worn his headset to try to reduce ambient noise, and sit in the seat with his eyes closed blanking out external noise as much as possible and staying oblivious to sight and sounds during the descent - as part of his executed plan.