Lad, what is your doubt? You claim to be a "sededoubtist" but you always like to argue that the normal SSPX/SSPX Resistance stance is heretical (except for Father Chazal who you claim to be a sedeprivationist even though he denies it). Doesn't that make you just a normal sedeprivationist? As if the other option is heresy there can't be a doubt. Or is your doubt possibly sedeprivationist possibly going back to the Novus Ordo?
I'm a dogmatic indefectibilist. I'm DOGMATICALLY certain that the Church, being guided by the Holy Spirit, cannot produce such a degree of destruction as would require good Catholics to leave the Church in order to remain Catholic. I don't agree with the IMO often-exaggerated notion of infallibility held by the dogmatic sedevacantists, but I also believe that this situation completely crosses a line. We're not talking about an isolated dubious statement. We're talking about a brand new religion that lacks the marks of the Holy Catholic Church. That is not possible. Imagine if there had been no New Mass, no bogus canonizations, etc., and the only purported error in V2 was that of Religious Liberty. Would there have been a Traditional movement? No, in that event, good Catholics who questioned that particular teaching would simply have respectfully raised their objections to the Holy See through the appropriate channels.
Now, HOW one wants to think out the problem that this poses for the papacy, that's a detail that has been disputed, whether one wants to go with the Bellarmine position or Cajetan / John of St. Thomas position ... which Father Chazal holds, there's freedom there. Heck, it doesn't matter to me if you want to speculate that Montini was blackmailed due to his sơdơmite lifestyle or else even that he was drugged, imprisoned, or replaced by a double. I don't even really care if someone wants to claim that all the V2 teachings and the NOM are somehow reconcilable with Tradition. I think it's preposterous, but at least it tacitly upholds the principle that the Church's Magisterium and Universal Discipline cannot defect.
Archbishop Lefebvre followed this EXACT same line of reasoning in that audio clip that has been posted here. I'll try to find it. He admits that this degree of destruction is not possible given the protection of the Holy Spirit over the papacy. Then he starts speculating about how this could have happened, whether Paul VI was drugged, which he didn't think was likely, or whether the See was vacant, and he did maintain that this was possible. But in the end he felt that he didn't have the authority to go there (very similar to what +Vigano has articulated) and "preferred to wait" for resolution of that question.
I'm sedeprivationist only in the sense that I believe the distinction to be quite valid, and that it works better than pure sedevacantism for a lot of reasons I won't go into here. But whether or not we are in a sedeprivationist situation is a determination made by our private judgment, and we cannot have the requisite certainty to dogmatically know this.
I believe that the dogmatic sedevacantists confuse dogmatic indefectibilism with a dogmatic assertion regarding the status of the papacy, and I feel that there's a logical gap there that they don't see.
If Siri was in fact legitimate pope, then the See wasn't vacant during that time.
There are a number of prophecies that refer to an "eclipse" of the Church, which refers to the fact that the true Church is hidden behind something else.