We do use all of their brain, 100% of it. We just don't use our brain consciously. We only consciously control about 10% of it.
The misinterpreted idea that I was explaining to you earlier is that you're not supposed to consciously use your brain 100% of time. Think about it. If you were consciously controlling your brain, your liver, your sweat glands, your body temperature, etc etc etc, don't you think that would be A LOT to think about and control? You would have no time to think about other things. That is why you do not consciously control 90% of your brain.
Also consider this, if you were able to control your heart beat, your liver, your sweat glands, etc etc, does that make you smarter? No. It just makes you work harder to keep your body in equilibrium, which will likely make you dumber because you have less time to form ideas and actually think.
No, it wouldn't. As I said, logic is based on the validity of the argument. A person dumb as a door nail that says 1 + 1 = 2 is correct, it's also correct if a person with an IQ of 200 said the same thing. Logic isn't based on intellect.