So when Paul tells the Church in Corinth to expel the sexually immoral (1 Cor 5:2), is that making their salvation dependent on “another mortal’s” (Paul’s) decision, too?
No. All that is doing is removing that person from that congregation. That person could still confess their sins and if they weren’t saved before could still come to the saving knowlege of Jesus later. The removal is discipline, which all churches have the right and responcibility to do, but they don’t have the power to “block” their salvation, which that Catholic Catechism teaches (as in saying that the Catholic church or communion is “necessary for salvation”).