"The accusation that the Church has erred is convenient....".  It may be convenient but that does not make it so for everyone.  To assume someone chooses to disagree with the church just because it's easier suggest that you're not really talking to the people who you refer.

"The corollary is that if the church can err...". Exactly, you can never be sure.  The level that one is sure is found in a prayerful relationship with God.  It develops over time with commitment to God.  The Church's rules are there as a means, not and end.