Sorry that I lost track of this thread.
Infallibility = the trees.
Indefectibility = the forest.
When quibbling about infallibility, we lose the forest for the trees.
Infallibility refers to specific doctrine or discipline that are guaranteed by God to be true and right (or at least not false and not wrong). Now, certainly, not everything a Pope teaches is guaranteed infallible. So R&R seize upon this to say that neither Vatican II nor the New Mass are protected by infallibility, and therefore could be false and/or harmful ... by minimizing the scope of infallibility. To combat this, then, the sedevacantists go to the opposite extreme by suggesting very strongly that the Pope is guaranteed infallible every time he passes wind from his lips or spills ink onto paper from his pen.
Unfortunately, the consequence of the R&R minimalism is that, for them, it's not inconceivable that 99.5% of the Pope's Magisterium could be completely wrong, harmful, corrupt, polluted, and leading souls to hell. But that consequence
ad absurdum violates the Church's overall indefectibility. If the Magisterium and Universal Church could become so badly polluted that Catholics jeopardize their salvation and harm their souls by submitting to it, then the Church's Magisterium and Universal Discipline would have FAILED, and the Church failed in her overall mission. That absurd consequence of their thinking is what the sedevacantists so violently oppose. And rightly so. Yet they feel the need, in order to combat this thinking, to exaggerate the scope of infallibility in the strict sense too far in the other direction. NO CATHOLIC THEOLOGIAN HAS EVER HELD EITHER OF THESE EXTREMES, that either the Magisterium as a whole could become substantially corrupt, overall unreliable, and harmful to souls ... nor that every utterance of the Pope is guaranteed inerrant by God. As in so many things,
in media stat veritas (the truth stands in the middle).
With Vatican II, we are NOT talking about an isolated statement in the narrative portion of a 30-page Papal Encyclical. We're talking about an entire Magisterium polluted with error and Modernist thinking. THAT is not possible ... even if it is possible, in the strict sense, for any given statement here or there to be false.
Let me quote Msgr. Fenton on this:
It might be definitely understood, however, that the Catholic’s duty to accept the teachings conveyed in the encyclicals even when the Holy Father does not propose such teachings as a part of his infallible magisterium is not based merely upon the dicta of the theologians. The authority which imposes this obligation is that of the Roman Pontiff himself. To the Holy Father’s responsibility of caring for the sheep of Christ’s fold, there corresponds, on the part of the Church’s membership, the basic obligation of following his directions, in doctrinal as well as disciplinary matters. In this field, God has given the Holy Father a kind of infallibility distinct from the charism of doctrinal infallibility in the strict sense. He has so constructed and ordered the Church that those who follow the directives given to the entire kingdom of God on earth will never be brought into the position of ruining themselves spiritually through this obedience. Our Lord dwells within His Church in such a way that those who obey disciplinary and doctrinal directives of this society can never find themselves displeasing God through their adherence to the teachings and the commands given to the universal Church militant. Hence there can be no valid reason to discountenance even the non-infallible teaching authority of Christ’s vicar on earth.
...
It is, of course, possible that the Church might come to modify its stand on some detail of teaching presented as non-infallible matter in a papal encyclical. The nature of the auctoritas providentiae doctrinalis within the Church is such, however, that this fallibility extends to questions of relatively minute detail or of particular application. The body of doctrine on the rights and duties of labor, on the Church and State, or on any other subject treated extensively in a series of papal letters directed to and normative for the entire Church militant could not be radically or completely erroneous. The infallible security Christ wills that His disciples should enjoy within His Church is utterly incompatible with such a possibility.
So the larger "body of doctrine" must always remain incorrupt. That is why a number of Popes taught simply that the Magisterium is infallible and inerrant ... viewed as a whole, from a broader perspective, considering the forest and not getting lost in the trees.