A. My copy of The Divine Comedy is heavily footnoted and indexed but I can find no reference such as Guest suggests. But there is a reference to shooting someone the finger.
B. The Plantagenets were responsible for several oaths. It was a family custom for each to swear by a different part of god's anatomy.
C. Modern English swear words arose because, to the Norman aristocracy, French latinate terms were natural and proper while their Saxon equivalents were "dirty" and low.