Should it be mandatory to use seat belts in a car?

Yes because no one is immune to the laws of physics. Sometimes you have to force people to have common sense. Because if they get badly injured in a survivable crash from not wearing their seat belts it is society that pays the bills.
I only support an exception if the person injured in an accident loses their medical insurance if they were not wearing a seat belt.

I walked away totally unharmed from a 40 MPH crash when a Tacoma ran a red in front on my Miata and I T-boned him. The car was a write off but between the air bag and seat belts I was fine. In the 60s I broke a windshield with my head in a much slower crash as the car did not have seat belts. My head hurt pretty bad and the damage to my forehead took a long time to heal.

I have been there and fortunately lived through it.

Yes.

The use of seatbelts is an issue that affects people beyond the drivers. Emergency responders have more to deal with if people did not wear seatbelts in a crash. The road is a collective compromise.

No, they shouldn't. Do I think it wise to not wear a safety belt? Certainly not. I do, however, feel that putting your own health/life at risk is your business, it's a matter of personal freedom.

Some insurance companies (where legally available) offer motorcyclists a "ride free, for a fee" policy, which allows them to ride without a helmet at a slight increase in cost. This could, and I think should, also apply to people who don't wish to wear a seat-belt.

YES! as soon as they enforce motorcycle riders in wear their seat belts.

Well if people are so stupid and do not care about their safety, then you need to do it by law.