I don’t see how the one issue relates to the other. I think it is important to realize that the Church was in place prior to St. Paul’s conversion. In light of that, it is not even feasable for St. Paul to have been the rock and the first Pope. Jesus did not rename him “rock” as He did St. Peter (when someone receives a new name from God, it is very significant in terms of a new beginning: Abram to Abraham, Sarai to Sarah, Jacob to Isreal). Jesus did not give St. Paul the keys to the kingdom of heaven and earth; He gave them to St. Peter. Jesus did not tell St. Paul that He was going to build His church upon him; He said that to St. Peter. Rather, Jesus’ first words to St. Paul were “Saul, Saul, why are you persecuting me?” We don’t know exactly how long St. Paul was persecuting the first Christians, but the fact that he spent any time at all persecuting the first Christians means that the Church was already founded. This is not said to take anything away from the work that he did in evangelizing. He was enormously important in the spreading of the Gospel (especially to the Gentiles), but that should not be confused with the founding of the Church. Jesus did that Himself before He was crucified and reiterated it after He had risen by commanding that St. Peter feed and tend His sheep and lambs.