And there is that pesky free will thing again,too.
If God doesn't allow men to fall into errors and sin there is no free will nor any merit for resisting temptation.Members of His church are still only men and can err. If the coming of Jesus was to somehow insure no one would err from then on,what is the purpose in letting us continue earthly existence. Free will or puppet,it is one or the other. Now I choose to believe and hope God chooses to grant man free will and also provide guidance and strength to choose rightly.
I try to hope that all will accept salvation even as I believe murderers and the like must be permanently stopped.
*I think you are conflating impeccability and infallibility. Jesus established His church as the Pillar and ground of truth. He sent the Holy Spirit upon it to teach it all truth.
Think of infallibilty as a "negative" charism in that our Triune God prevents the Magisterium - the Pope and the Bishops teaching in union with him (esp in an Ecumenical Council) - from teaching error.