Yes, this is correct, but what theologians (Billot, Gueranger etc treat this expressly) teach is that such doubts can only be raised up until the point when a single claimant has universal acceptance (for example, during the GWS, so long as there were two or more putative Popes having widespread acceptance, the status of any one Papal claimant could arguably be said to be positively doubtful, but certainly not after the point where any one had universal acceptance.). After a man is universally recognized as Pontiff by the bishops, all conditions for the legitimacy must be held to have been fulfilled.
The election of a heretic cannot be convalidated, it is true. But if a man who is accused of heresy is universally recognized as Pope, as is relevant for our case, that suffices to show that the accusation is incorrect. These theologians teach that if a heretic were ever elected, or a Pope ever became a heretic, it is an absolute impossibility that he will have universal acceptance.
Modern sedevacantists will not give a straight answer as to the question of why the legitimacy of Pope Pius XII cannot be questioned - the correct answer is because Pope Pius XII had universal acceptance. This case is taught expressly by Van Noort.
This principle in the abstract is certain, though its application in the concrete can be disputed. Cardinal Billot gives an excellent illustration of the application of this principle, in the case of Savonarola, who accused Pope Alexander VI of not being a Christian and therefore not being Pope. Cardinal Billot affirms that because Pope Alexander VI was universally accepted as Pope in that moment when he was accused of being a heretic, he was certainly Pope, and therefore not a heretic, at least not formally, and therefore did not lose the ordinary and universal jurisdiction by which he was and remained the Pope.
I agree with your analysis regarding the problems posed by sedevacantism. I myself have referred to the Pope-Sifting problem.
I do think that the "universal acceptance" thing is problematic though. What does that mean? You look at the various polls that have been taken, and it's quite obvious that at least 80% of the people who claim to be Catholic are in no way, shape, or form Catholic. So, to me, just because he's been widely accepted by a group that appears to have largely lost the faith, does that constitute "universal acceptance" by the Church or by Catholics? I've seen some estimates that 90% of the Christian world went Arian during that crisis, and these would undoubtedly have accepted an Arian pope.
Let's look at another case. We have a pope who is elected that lapses into heresy. At first he's universally accepted by the Church, but then an inner circle of Cardinals finds out he's a heretic. Before they are able to go through the process of declaring him a heretic, and of promulgating this finding, he continues on as universally accepted. But, according to the St. Robert Bellarmine opinion, he has already ceased to be pope.
You look at a Pius XII and he clearly had what could be called universal acceptance. Once you get into Paul VI, John Paul II, etc. you have a significant Traditional movement that questions whether they're Catholic in any real sense. I just have a gut feel that the cases of Pius XII and Paul VI et al. are different, if based on nothing more than the fact that the latter wreaked absolute havoc upon the Church.
I balance the considerations you raise with my inability to accept that a legitimate Pope could teach heresy to the Church and promulgate a gravely defective Rite of Mass. I don't consider the latter to be possible without undermining the indefectibility of the Church and rendering it completely meaningless. While I don't agree with a lot of sedevacantists that every
obiter dictum in any encyclical or allocution constitutes infallible truth, I do feel that V2 and the Novus Ordo Missae have crossed a line, a "red line" as it were. I find the latter more problematic than the problem of a heretic pope.
Consequently, I consider myself a "sede-doubtist". Plus, there could be other explanations such as the fact that Paul VI was blackmailed (if certain well publicized allegations about him are true) ... which would have rendered his acts (V2 and the NOM) null and void.
I was reading on another thread where someone (may have been Cardinal Billot) stated that the legitimacy of papal elections is determined by the positive law laid down by the previous pope(s). If an election was illegitimate, then I don't see how this can be "convalidated" by the Church.
I still think that the most plausible explanation is the Siri election in 1958 along the lines of the "uncanonically elected pope" in the prophecy of St. Francis.
But I cannot judge either way with the certainty of faith, and the legitimacy of a pope must be known
a priori with the certainty of faith, so I am not competent to judge on the matter but try to do my best based on the fact that I am in doubt about their legitimacy. If I had the certainty of faith that they were legitimate, then I would find a way to submit without compromising my faith. Otherwise, Traditional Catholic theology also states that to be in a position like ours would be schismatic.
Let me ask you, Nishant, do you have the certainty of faith that the V2 Popes are legitimate? I know that I don't. Even Archbishop Lefebvre didn't seem to have that certainty. He speculated about possible illegitimacy many times. That by itself indicates that he didn't have the certainty of faith regarding the legitimacy of the V2 popes.