>> When one of their leaders - namely Williamson - asserts that the Holy See teaches heresy, then he himself is teaching the heresy that the Church is defectible. <<
I’m not sure about Williamson, but I believe the notion is that Vatican 2, and many actions by the Pope afterward, may contain elements of heresy, but don’t violate the notion of infallibility because there was no invocation of infallibility: The objectionable portions of Vatican II were explicitly non-dogmatic in nature, and Paul VI’s abolishment of the Latin Mass (can we please call a spade a spade? The Latin Mass was abolished by Paul VI!) invited heresy, but was an act of governance, not of establishing doctrine.