Why would you do it? The pregnant abdomen is such a strong symbol of youth, health, life and hope that it should not be a problem to anyone. Tight binding of the uterus is not only uncomfortable but also potentially restrictive to the fetus.
Yes. There is no problem with wearing whichever clothes you want when you are pregnant. We normally advise looser clothing because it is more comfortable. Some women will have nausea or even pain with binding clothing. However, that is a comfort issue for you, not a safety issue for your pregnancy. Tight clothing will not restrict your baby's growth or movement.
Yes. Just make sure they are comfortable and not putting too much pressure on your skin.
Wear what you like. You can wear what you like, as long as it is not tightly restrictive. I doubt there's any evidence to suggest danger with any particular clothing, but it makes sense to not compress the belly too much.
Yes. It is perfectly normal and natural for a woman's shape to change during her pregnancy. So i wouldn't recommend any garments which are constricting around the abdomen, particularly once you reach the second trimester onward. Remember, there is a baby growing in there and they need some room!
Yes. Yes, spanx and varieties of that are ok. Just make sure it is not cutting off your circulation.