Correct. Five clicks of the sleep/wake button will disable Face ID temporarily. If you're prone to keeping incriminating stuff on the phone, you should probably have it off entirely.
There's no "if" statement required. Everyone should disable their phone when encountering law enforcement unless they're using it to record (and it's a burner). Who knows how corrupt that particular LE is and what agenda they are following.
Yes. People more likely to encounter law enforcement in surprise, break-down-the-door scenarios where the cops might have a vested interest in keeping you away from the phone should probably forgo Face/Touch ID entirely, though.
You can also just hold power + a volume button until you get the "power off" dialog, which also disables FaceID until the next time you enter your passcode.
Which is why I would never enable using my face to unlock my phone.
It's probably the only thing I would ever call an insecurity feature.