1) It's actually not that expensive, but the regulations made it so. I remember something from titans of nuclear or some Jordan Peterson podcast. I'll try to write the gist of it here:
There was some rule, that the cost of safety (like how thick concrete should be in some places), could be so high, that the usually cheaper fission energy would be equal in cost with the other sources (like burning oil). Then came the oil crisis of the 70's in USA. The safety margins got boosted to crazy levels, without any realistic gains. Moving from 99.999% to 99.9999% safety (just an example).
When the oil prices dropped, safety standards stayed and now fission energy is expensive. At least in USA and EU. Not in France or South Korea, which streamlined the regulations.
2) not with the modern technology, it isn't. And there are even safer alternatives like marble balls reactors that can't meltdown even if cooling is shut down.
3) not using it is bad for the environment. Fuel requirements are minimal compared to other plants. Even some types of renewables pollute more per W of energy produced. Like wind turbines that will fill up landfills at some point.
4) Thorium reactors. If we just give the fission energy some research & development, we can burn all the spent fuel up in thorium reactors.