3. That claim is yet to be proven. Even if the trades don't go away, legalization will deter criminal activity because people will no longer have to resort to illegal means of obtaining pot which, as you will learn in Economics class, increases the price. I've yet to hear of a single case of the black market selling something for less than it would cost in a free market.
Haven't you heard of cigarette smuggling or moonshining? If you set a sin tax high enough, it will encourage people to engage in the black market to evade the tax.