Your question requires 3 separate answers, so I'll take each issue in order. First, if you believe as a Christian that Jesus is the Son of God manifest in a physical body, then it follows that he cannot make a mistake, because if he could, he would not be God. If you do not accept the core tenet of Christianity, then it would not matter whom Jesus chose as his Apostles. Second, Paul was not one of the original twelve apostles. Paul was a Roman named Saul who had a vision and was converted to Christianity after Jesus had been crucified. Third, it seems that Paul was necessary since he is the one who is credited with bringing the teachings of Jesus out of the Jewish community and making them available to all people. As such, only one Paul seems to have been necessary to get the job done.