You can switch them off but only until the engine is turned off again. Most manufacturers have a shortcut on the dashboard or steering wheel though. Eventually you just get used to doing that every time you start driving.

Depends on the car (and the regulatory regime, I'd imagine). My fancy pants 2025 car is happy to leave driver alertness detection disabled, which is handy because it's not good. Of course my ultra base model 1981 van doesn't have any features... it's a lot more fun to drive, other than the engine noise is pretty oppressive on a drive any significant length oh and the floor is missing where the accelerator pedal should mount :P

There'll be firmware hacks to force that mode soon enough.