You would need to exhale if the roof doesnt just implode and leak out all the air and you take a breath when the pressure between inside and outside of the car is equalized. You wouldn’t be able to open the door before water rising from the floor compresses air inside the car enough. Modern cars feel quite capable of keeping some air in them when the car sinks.

Nope, unless you are worried about stalkers.

Real shame that GrapheneOS only supports google pixels since they are kinda expensive.