If you drop below 100g of carbs a day, your body converts protein to glucose as well.
So dropping below 100g of carbs doesn't mean your brain won't function properly, but some people do better with low carb than others.
Um, no. If you eat protein then your body can convert that to glucose. You don't need to burn off muscle in order for this to happen.
It is important to get sufficient protein when you go low carb so your body has enough to fuel both your muscles and your brain, but cutting carbs does not mean your body is cannibalizing your muscle.