With all that said, I believe that Vatican II would constitute essentially a defection of the Magisterium, and the New Mass would violate the infallibility of the Church's Universal Discipline. It's easy to miss the forest for the trees.
What has happened is that R&R posits that the Magisterium could have defected and the Church's Universal Discipline defected, so the Sedevacantists counter with the opposite extreme, an exaggerated notion of infallibility whereby the Pope is infallible pretty much every time he puts pen to paper.
Is this defection of the Magisterium in regards to the infallible teaching of EENS?
But of course. It's the key error behind all of Vatican II. Yet the Magisterium cannot defect. I was saying that R&R would posit a defection of the Magisterium. It's one thing to find an error here or there in some obiter dictum but not a wholesale denial of EENS by the Magisterium.
The Pope can't help but be protected by the Holy Spirit when he is teaching to the Universal Church on matters that deal directly with faith and morals. Its that simple, so in a sense every time he puts his pen and paper to the above conditions. You must give assent of faith, its that simple. That is the teaching of Vatican I, and not just Vatican I but so many other Dogmatic decrees that can be found in Denzinger that precisely teach this.
The tactic that unfortunately many use, is to reduce the Church's teaching in order to make these men minimally Catholic. I never did this as a sedeplenist, yet it is extremely common. There is only one of them that can be right, either all the previous popes were wrong, or the current "magisterium" is right. It is either/or, you can't pick and choose which one is right and which one you decide to follow. Doing this leads to even more heresies, because now instead of having just one man who is protected with infallibility, you have every single believer have infallibility when privately interpreting magisterial docuмents (usually Denzinger that people use). Instead of reading things with the mind of the Church, you proof-text and deny Church teaching, downgrade it etc... This is definitely not a Catholic attitude that is a common method of exegesis by traditional Catholics.