You should take an appointment with his cardiologist and ask all those questions directly.
My wife had open heart surgery in 2019 (arterial valve stenosis...had a valve replaced). Am familiar with some of the issues.
I would not take general advice from people like me who know a bit of this and a bit of that...
And everyone's case is slightly different.
I would go to the source and get information directly from his cardiologist.
As to your question of how long can people live in a state of diminished capacity? Well, obviously, the cardiologist is going to give you the most accurate information. But, I will say, generally, that people can live in a diminished state of capacity for a long time.
Some medical issues degrade and get worse. They either get worse slowly or more quickly. Other medical problems are not like that...they do not really get worse over time. They tend to stay roughly the same.
So sorry about the difficulties. We are still dealing with many issues here. Very difficult.
Take care now...