Captain Key may well have been dead before PI hit the ground. He was certainly affected by his coronary artery tear and probably incapacitated during the final few minutes of PI flight.
However, I do understand your point. Both Capt Osbon and Key had illnesses which became critical to their performance during flight (to state the bloomin obvious) Both must have had some disease progression before the condition became critical. Key certainly did, as he had had a previous cardiac event caused by underlying atherosclerosis. Osbon probably had some mental affectation prior to him boarding the aircraft.
The moot point is whether Capt Key or Osbon actually understood they were ill and thought to seek medical help. Also it may not have been reasonably foreseeable to either captain that their illnesses were overcoming them.
Medically speaking, the two captains are in two different eras. With Capt Key, his condition may well have not been spotted by the most diligent cardiologist of the time. Today it would be likely that the potential results of atherosclerosis would have been identified (he would equally have been at risk of a stroke) and the previous healed MI would have been identified. Mental health care has also improved dramatically.
As for prosecution - why not? Court is the place to establish the material evidence of Osbon's mental capacity and his awareness of his mental illness. The facts (and expert opinions) will be revealed there and judged upon. If he is innocent under the law then the verdict will vindicate him, even if now he is mentally well. Of course Key is dead so no trial would be possible.
As for the captain who had a teddy bear hanging in the cockpit - may not exactly be SOP but if his rationale is that it helps to get female crew to sleep with him then that is a perfectly solid rationale. Nothing in the slightest bit irrational about that. If, however, he had said that the teddy bear gave him clearly audible commands to fly his aircraft then there would be a cause for concern.