There rarely are rational thinking people in the room.
Rational people would have very little need for government at all.

There really is a "slippery slope" in human psychology, but it goes by terms such as "cognitive inertia" and "confirmation bias". Once people get an idea stuck in their heads, they tend to go with it (to their deaths, at times) even when all logic points the other directions.

Yes, it does.
I am not a chemist or biologist, but my understanding is that sunlight starts (or facilitates) a chemical process that produces Vitamin D. Without sunlight, that reaction does not happen.