The comparison of the infallibility of Rome to the pronouncements of Griswold is laughable and spurrious. The problem with Griswold is that he contradicts 2,000 years of Christian tradition and scripture. Infallibility is the opposite tendency: Once spoken, a doctrine can never be rescinded. It is not a liberal impuse, but the ultimate preservation against liberal impulses.
You miss the point, classical Anglican doctrine has one FINAL authority, God's Word, the Scriptures. Roman doctrine has twin final authorities, authoritative Church teachings, AND the Scriptures. Liberal Anglicans/Episcopalians also have twin final authorities, Church speculations...(often never more than that...)and (supposedly) the scriptures.
You're right the nature of those "authorities" are completely different (the liberals make an ultimate claim of no ultimate authority....) however, both systems, liberal Protestant, and Roman Catholic, do not have the scriptures alone as final authority.