1. accidents happen so quickly, that most reactions are instinctual, like a nerve impulse taking a direct route up the spinal cord as opposed to the typical neuro-pathways, such as when jerking back from touching a hot object to avoid a burn. As such, the immediate reaction is to protect the face and head with the forearms in a crash. You don't usually have time to ponder the fact you're wearing a helmet. Survival instincts take over.

2. this one I don't quite follow.

3. this is a matter of common sense, and one's level of mental development. They say one's ability to make a rational decision isn't fully developed till your mid 20s. This is why kids do stupid things in cars, for example. That said though, sure I could see this being true, that someone would take a greater risk. Same with wearing 'body armor' or those exo-skelital outfits you see BMXers wear when stunting.

4. This goes back to my initial comment. A helmet protects the head, not the neck. snapping your spinal vertibae is not what a helmet is to protect against. As you said, it's to prevent a rock from causing a subdural hematoma for example, and even a glancing blow can cause this type of damage without a helmet to absorb the impact. This is also why once you wreck with a helmet on, throw the helmet away and buy a new one. You have no way to know if there was structural damage done to the helmet, even with no visual damage.

5. sure. I never said a helmet will save you from being hit by a car.. lol again, the helmet is there to prevent head trauma. period. It's not an end all be all device to protect you from death or injury. It's another layer of protection in case you Do wreck. The same as a tail light, a yellow jersey, or a mirror can help you to avoid an accident. One more layer of safety.

As for drivers giving more room to someone without a helmet however, I'm not sure I agree here, for the vast majority at least. Granted we're stereotyping here, but I've found there are usually two types of drivers. Ones who give ANY cyclist or pedestrian a wide berth, and those who flat out don't appear to give two sh**s and don't move over at all.. even finding it funny to hit large puddles and such to make riding that much more miserable when near a road.
I don't think a helmet plays much of a factor there either way, but again, this is just from observation over 20 years or so. Nothing to substantiate that.