Yes they can do. Whether they choose to or not is personal choice and can also be determined by the environment. For example, if you choose to do that in a public place, you would keep as many clothes on as possible really, whereas in a private room you could take it off or keep it on.
Generally people would probably remove shirts as they could make them very sweaty, or they could get crumpled, and also because it gives their partner more access to their body. It is not unusual to keep shirts on though and is all about convenience and personal preference.
Yes, if their nipples get cold.
People have sex with all kinds of things on, from cotton, nylon, latex, rubber. Try humping with hemp leg warmers. They say it's fantastic.
yes if you are in one hell of a hurry or you just hate the way your body looks
lol umm i guess if their in a hurry or sumthing