Ok, I think that we are starting from the same point.
Now, I do think that the Holy Spirit does protect the Church. I do use the passage about the gates of hell not prevailing. I add to that, as no single passage of Scripture was ever meant to stand alone, with the part where Jesus promised that the Holy Spirit would guide them to all truth. I also see where Jesus promised to be with us always, even until the end of time. That all leads me to think that the Church cannot err when it comes to faith and morals.
I also have another angle on it. The Church is the Body of Christ. Now, we know that Christ was sinless and perfect. Could the Body of Christ be perfect? Yes and no. As it is made of sinful me, it can never be perfect in that way. However, is it crazy to think that, like Jesus, she is always perfect in teaching?
The Church is also the Bride of Christ. Same concept. She is spotless only in the sense that she teaches correctly.