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But regarding the preconciliar popes, there is no doubt as to their legitimacy at all (i.e., because they never attempted to teach doctrinal error to the universal Church).

And I agree with you completely.  There is in fact zero doubt about the preconciliar popes.

But if we have to ascertain certainty from the orthodoxy of their teaching, that creates a strange convalidation feedback loop where we can't know a priori whether they're legitimate or not.  So who determines whether they've taught doctrinal error?

What if Pius IX was in fact wrong when he condemned religious liberty?  How do we know he was wrong and the V2 papal claimants were right?

And I agree with you completely.  There is in fact zero doubt about the preconciliar popes.

But if we have to ascertain certainty from the orthodoxy of their teaching, that creates a strange convalidation feedback loop where we can't know a priori whether they're legitimate or not.  So who determines whether they've taught doctrinal error?

What if Pius IX was in fact wrong when he condemned religious liberty?  How do we know he was wrong and the V2 papal claimants were right?

I think you misunderstood me:

Certitude regarding papal legitimacy comes from universal assent (for both pre- and postconciliar popes).

The reason the TYPE of certitude is different for each is because the latter teach error, but the former did not.

That teaching of error on the part of conciliar popes reduces ever so slightly infallible certitude to moral certitude, opening the door a sliver to the theoretical possibility -however unlikely- of sedevacantism.  But this "sliver" (i.e., the reduction from infallible to moral certitude, in light of teaching error) no more equates to a positive doubt or doubtful conscience that the state of a man having just made what he hopes is a good confession:

He has no infallible certitude his absolution was valid, but THAT DOES NOT MEAN HE DOUBTS IT: Having confidence he has satisfied the requirements of making a good confession, he has moral certitude he has been validly absolved.  He is not in a state of doubt, and if the devil irritates his conscience to try and create doubt, the penitent quickly and persistently disregards the intrusions.

This analogy fits the mindset of Archbishop Lefebvre perfectly.

You like logic, right? Well, it seems to me your doing something like violating the law of the excluded middle: if these popes are not the Magisterium, the seat is vacant; if they are, its not, and all the conditions of the Magisterium apply (indefectitility, etc.).

You just keep repeating this over and over despite the numerous times I've explained it.  This is completely false.

All that is necessary to hold is that the Catholic Magisterium cannot go off the rails as badly as it has.  There can be numerous explanations, given that constraint, which do not violate the principle, including the assertion made by XavierSem and others that the Magisterium has NOT in fact gone badly off the rails.  You could adopt the attitude of a Bishop Schneider that there are only a couple minor tweaks needed to reconcile Vatican II with Tradition, and the rest is merely a question of Modernists spinning some ambiguities in their favor.  That position, to be quite honest, is less offensive to a Traditional Catholic understanding of the Magisterium than the R&R promoted by Johnson and other (evidently also yourself lately).

It's also IMO very possible that Montini was being blackmailed, so that the various acts of his were not entirely free and therefore would not have constituted legitimate Magisterium.  Montini has been credibly accused of both sodomy and of being a Communist agent.  There was in fact a group of Communists at Oxford who were known to be a "honey pot" operation to lure in and then blackmail ɧoɱosɛҳųαƖs.

There's the position of Father Chazal that, while the See is not vacant, it has been deprived of all teaching authority due to the heresy of the papal claimants.  Finally, there's the sedeprivationist theory (very similar to Fr. Chazal's) that the See is materially occupied but lacks formal authority.

But of course the Magisterium proper is just the tip of the iceberg.  You also have the defective quasi-Protestant form of worship that many Traditional Catholics hold is offensive to God and a "Great Sacrilege"?  Really, the Holy Catholic Church could promulgate and implement as its normative form of worship a "Sacrilege".  Either you go the XavierSem route of claiming that it's merely less perfect (but not positively defective, harmful, and displeasing to God) or you must decide that this is not the work of legitimate Catholic authority.  To claim that the Church's public worship is Sacrilege is in fact a blasphemy ... and in fact a proposition anathematized by the Council of Trent.

Finally, you throw in the canonization of Montini and Wojtyla ... thereby polluting the catalogue of saints with two of the biggest scoundrels to every (materially) occupy the See of Peter.

There's no recovery for the Church from this kind of smear against it.  None.  At that point the Church has lost all credibility and has defected.