Picking a multivitamin, the first thing is to feel confident that what's on the label is actually in the product. I usually pick one from the pharmacy. I think a multi by a drug manufacturer is more likely to be legit than a bodybuilding brand where there is zero oversight.
Totally inadequate. The average balanced "diet" is already deficient. Depleted crop soils, industrial pollution, carcinogens, free radicals does a number on the human body. Then you have an epidemic for the over consumption of processed/fast/junk foods. Calorie dense, nutritionally void.
No one should overdose on vitamins or minerals, but taking enough to ensure optimum levels is wise and pretty cheap insurance.
It's pretty complicated when one really looks into those things you mention, beyond the headlines and memes.
Processed doesn't necessarily mean worse. Natural doesn't necessarily mean better than artificial in every case. Everything can cause cancer, including breathing. Dose determines the poison, everything can be poisonous.
Apples contain a natural known cancerogen, as do other veggies and fruits. Not all known crops are worse nutritionally than in the past. And on and on.
Do you use antioxidants? If so, which ones and why?