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"So this matter was not settled prior to VCII. The Council’s work indeed confirmed the opinion of modern theologians and authoritatively settled the question. On that basis, the subdiaconate and the minor orders were suppressed. Such a momentous decision inexorably altering the Church’s discipline must therefore be considered infallible."

Your man is here talking complete nonsense. The subdiaconate was not suppressed by the Council at all so this issue cannot be a test of whether V II made any infallible decisions. The subdiaconate was actually suppressed by Paul VI in 1972 in his apostolic letter Ministeria Quaedam i.e. 7 YEARS AFTER THE COUNCIL CLOSED.

There certainly were infallible decisions made by the Council in the decrees that were issued in Sacrosanctum concilium and Christus Dominus. Decrees are disciplinary in nature and not doctrinal. The whole point is that the Council made no DOCTRINAL DEFINITIONS - again as Ratzinger has already pointed out.