There can be a quite lengthy answer to that, especially since you're asking several different questions. But to concentrate on the main point: I am not wearing a helmet while driving because our cars already have a "helmet" incorporated into them. It's threefold: the outer metal casing that crumbles during an accident, a seatbelt, and an airbag. I am much safer inside the car even at high speeds than on a bike on lower speeds (compared to each medium).
Here again, you are using degrees of safety, as am I, it's just that our levels of risk tolerance are different. It is my contention that if you feel driving is in the least bit unsafe, in relation to head trauma, then you should be wearing a helmet at all times while in a car. I would still like to know the overall numbers of head trauma in a car versus a bike, and I still maintain that the number in cars is vastly higher in overall volume of head injuries. It should not matter that they are made with a "built in helmet", nor that the percentage of injuries on bikes versus cars is proportionally higher, because head trauma in cars still happens on a regular basis. You simply feel that it is an acceptable risk that you will not be one of the injured, I maintain the same when I ride my bike. Levels of risk tolerance and natural ability/skill/balance on a bike.