Read all your questions and to sum it up, the answer is simple: there's not one single exchange that allows USD trades into altcoins simply because cryptos can be created easily, and if that happens, imagine these exchanges being a hotbed for cash withdrawals from the devs of the coins and those supporting the coins itself almost immediately. With bitcoin being the currency these alts are pegged at, it is to ensure that the amount of money flowing out of an exchange is controlled and the exchange not operating to a loss. I mean, as an exchange operator, why would I let other people trade a relatively unknown altcoin with fiat knowing that it could just be another fraudulent way for the devs and his conspirators to make money?