If you’re one of the people tasked with enforcing the law, then yes you are not expected to know it. If you are a mere citizen, then you are expected to know the law inside and out.

In theory, no. In practice, yes. I think it's generally fine for people to be able to use discretion in a lot of cases. That discretion is not always evenly applied, but there are plenty of laws that the average person doesn't know and has no reason to even think about. I'd rather live in a world where the justice system is able to give people leniency and the benefit of the doubt than otherwise. If one of your brake lights is out it's possible you have no idea. Technically it's illegal to be driving. Most people would agree that it's a good thing that you can be given a warning or something instead of have to face the same penalty as a person knowingly driving with the same issue.