First let me say I am not a sedevacantist, I simply do not know. Indeed I sympathise with them, and consider them as faithful traditional Catholics. Explaining themselves is OK by me, but trying to get others to become sedes is a step too far.
My first encounter with the Dimonds was reading their defence of the 1741-1835 rejection of the 1616 papal decree defining and condemning Copernicanism formal heresy. The reason they did this is because by the same sedevacantist reasoning, every pope, from Benedict XIV in 1741, who accepted that heresy had to be an anti-pope. That is just not practical.
Sedevacantism, to numerous Catholics, is the only explanation left to them in the wake of the obvious errors of Modernism arising at Vatican II and thereafter by one pope after another. It is the only explanation left in order to remain Catholic in a time when the Church of Christ is saying and doing things that have been defined and condemned by popes throughout the history of the Church.
I must say I was under the impression Popes John XXIII and Paul VI had opted OUT of giving Vatican II their infallible perogative. The video here seems to show Vatican II had the infallible perogative. More confusion, but showing me how complex Catholic theology has become in these dark days.
Now I have long thought about this infallibility question as many others I am sure have also. As a result I have read much, pondered endlessly on things like Our Lady's words at La Salette and even the Scriptures telling us the antichrist will gain control. As a 50s and 60s Catholic when there were no traces of Modernism to be seen to the pew-sitters, we believed God would NEVER allow such scandals into His Church by way of Modernists, child abusers by nuns, brothers and priests, paedophiles, ɧoɱosɛҳųαƖs, etc.
Over the years I have witnessed Catholic theology getting more and more complicated, all trying to give Catholic reasons for allowing a defined and formal heresy of Copernicanism to become orthodox and a model for a new Catholic exegesis and hermeneutics. I have now come to a conclusion that satisfies me on that matter and I will now apply it to the subject of this thread's question of infallibility.
The answer I found in Vatican I docuмents on the infallible magisterium, and I will understand them in my own way, not based of a 1000 other theological versions of it. Here it is:
For the holy Spirit was promised to the successors of Peter
not so that they might, by his revelation, make known some new doctrine,
but that, by his assistance, they might religiously guard and faithfully expound the revelation or deposit of faith transmitted by the apostles.
Infallibility is attached to the ORIGINAL DOCTRINES AND DOGMAS defined and declared throughout time.
This being so the above teaching of Vatican I shows it is REMOVED from any pope who on any occasion tries to make known a new doctrine that CONTRADICTS OR IS DIFFERENT from the original infallibly defined doctrines.
This being so, and applying my interpretation (rather than sedevacantism) of above vatican i decree to Vatican II is that anywhere previous doctrines or dogmas are denied or contradicted no infallibility is present, and anywhere previous doctrines and dogmas are approved infallibility remains attached.
No doubt I will now be shown that I too have it all wrong.