Well, Ladislaus, you raise many points, which would require a detailed response. Even supposing doubt remains about universal acceptance, (we'll come back to that,) there is a distinct reason, as you noted, and as we've discussed, to think a 55+ year ongoing sede vacante is not possible, about which I think we can have a certainty of faith, or at least certainty proximate to it. After that, concerning Vatican II, the New Mass etc, it is only a matter of determining carefully in what circuмstances a Pope can be resisted, because if there is an independent reason to think such an interregnum is not possible, then it comes down to who is right between traditional Catholic SSPXers and indultarians, sedevacantism being excluded.
1. There are only 13 Ordinaries still alive who were appointed by Pius XII - none of them still in office - and these Ordinaries, according to St. Robert and Cajetan both, are the ones who must pass the necessary juridical judgment in the case of a heretical Pope. After that, the Cardinals or Roman clergy can elect another. Leave aside the doctrinal problem such an inability to perpetuate formal Apostolic Succession without the Petrine Succession poses, what about the practical problem of who the Pope-heretic is to be denounced to?
I believe both the canon on perpetual Petrine succession and a condemned proposition from Constance that "there is nothing to show that the spiritual order requires a head who shall continue to live and endure forever with the Church militant" preclude absolutely the eventuality of such an ongoing interregnum in the current circuмstances.
If I believed it was possible we were in the middle of such an interregnum due to the loss of office for heresy, I would immediately try to write to or contact the existing authorities, and try to convince them of it. For those who hold the opinion of John of St. Thomas or Cajetan, such a decision would be absolutely necessary. Even in the case of St. Robert's opinion, it is necessary, although it would be purely declaratory. Only after this, the Roman clergy can elect a new Pope.
S.B Smith in Elements of Ecclesiastical Law summarizes the state of the question like this,
"There are two opinions: one holds that he is by virtue of divine appointment, divested ipso facto, of the Pontificate; the other, that he is, jure divino, only removable. Both opinions agree that he must at least be declared guilty of heresy by the church, i.e., by an ecuмenical council or the College of Cardinals. The question is hypothetical rather than practical”
Another point of critical importance he mentions elsewhere in the same work,
"Laymen are not competent judges in matters of heresy, even as to mere questions of fact".
I think all your instincts are in the right place, because you've yourself mentioned in the other threads that something like this would have to be done, my question to you is - if we're in a 55 year interregnum, how can this be done?
2. Most modern sedevacantists do not believe the Roman clergy can never cease to exist, which the Fathers teach and in particular St. Robert himself believed, which Pope Sixtus IV taught and which following him all theologians teach, as Msgr. Fenton shows in an excellent article on the subject. Most do not believe the hierarchy can never cease to exist, as Vatican I taught "just as He sent Apostles, who He chose out of the world, so also it was His will that in His Church there should be shepherds and teachers until the end of time." Theoogians understand this to mean, as it says, bishops with a mission from Christ and through Peter, sent by the Pope and possessed of real governing power, or jurisdiction, will always exist, the CE explains this as well, I will provide some citations if you wish.
There is no problem, no doctrinal problem that is, with 98% of the hierarchy losing their offices, the same for the Roman clergy. But fully 100% of them absolutely cannot, that is impossible. These are the only authority in the Church, through whom the Catholic Church retains formal Apostolic succession and Her Roman link, both of which depend on the Petrine succession, deriving their authority from past Popes.
This brings us to sedeprivationism, which I know you favor, and which is perhaps the best response to a perpetuation of succession, at least material. But whatever may be said the possibility of a "material" Pope possessing only the power to designate but not governing authority or jurisdiction, for the reason mentioned earlier, the same is not possible that in the whole Church, that is, among all bishops together at the same time. The whole Church cannot be in a state of potency without actual power or have all Her bishops deprived of pastoral and governing power at some time, if it has, then the Catholic Church has ceased to be Apostolic, which is impossible.
So I ask - do you believe otherwise, that governing power or jurisdiction, which is the form of Apostolic succession, can cease to exist in the whole Church? And if not, does not indefinitely extended sedevacantism as well as sedeprivationism seem problematic because it leads precisely to that?
3. Finally, we come to another theory I know you favor, sede-impeditism or Sirianism - I understand the attraction of this thesis, but it seems to me Cardinal Siri's "claim" doesn't even rise to the level of a doubt, because it is not even a claim, nor accepted by anyone and according to you was not even accepted by or known to him, and to him would apply in that case "a doubtful Pope is no Pope". The assertion that such a man would remain Pope, because his resignation was made under duress, is precluded by the unanimous teaching of theologians indicating it is impossible that a man can somehow internally retain the pontificate without knowing it while together externally with the Church accepting another man as true Pope. The universal acceptance of the other claimant would mean, at least from the moment it was verified, no further doubts could remain.
I understand you are not convinced of this point, and I actually think you are on the whole much more consistent than a few of your critics on this thread, who castigate you for not holding to the universal teaching of theologians, but then do that themselves. But let me give you a simple proof these theologians give, for why it proves a Pope is infallibly Pope, and not a heretic (at least not a formal heretic) in particular - if the whole ecclesia docens professes communion with a man as Pope, then, if the Pope were outside the Church, the whole ecclesia docens or Church teaching would also be outside the Church, which is absurd. Therefore, the converse is true, that their recogntion of him in this sense proves that he is inside the Church, is the Pope, and not a heretic. The people who don't accept the teaching, on the authority of the theologians, should at least try to address this argument they give as its proof.
That brings us back to what universal acceptance means, and whom it includes. As I said, if 98% of the episcopate has lost their office, the other 2% would be the only ones who would be considered. But that doesn't mean anything, because fully 100% of the ones who can even be considered for this are the ordinaries appointed by the Popes, by Pius XII if you believe he is the last Pope. Canon Law says only the Pope can institute a bishop in a vacant diocese, even if he is designated to the episcopacy by another, therefore the only diocesan bishops or Ordinaries to be considered are those appointed before Pius XII died. And if all of these who still retain their offices accept the Pope as Pope, then the Pope is infallibly the Pope. I can show you the quotes, from John of St. Thomas and others, who say that the universal acceptance of a Pope is a sign and infallible effect of a valid election, and is immediately recognizable as such.
Taken together, these two points prove to me beyond reasonable doubt that the sedevacantist explanation of the crisis in the Church is mistaken.
I will write a critique of the indultarian position later, also relating to Vatican II and the New Mass, since you've said in the past, and also on this thread, that if you were convinced the Pope was the Pope, and that SH was correct, you would be an indultarian, rather than agree with Bp. Fellay and the Society.