I hold that both John23 and Paul6 were not legitimate popes (anti-popes, non-popes, whatever you want to call them) spiritually speaking. They may have been validly elected, but only for the physical/govt/human office (but that's another topic and besides the point).
But...even if they were valid popes (temporally and spiritually), that V2 was not infallible, not binding (in the normal, historical sense of the word) and not unquestionable.
Why do I argue this? Because I believe that God would not allow even anti-popes to *appear* to teach formal error. (Formal teaching = apostolic authority, doctrinally binding, under pain of sin, loss of salvation).
Evidence of this are the many indult communities who accept the pope as legit, but who are still allowed (by both law and the various admittances by new-rome officials), to question/critique V2 and also the new mass. If any group of catholics would have to swallow 100% the errors of V2, it would be the indulters (since the typical novus ordo type could care less about V2 or religion in general)...but, they are allowed to question this liturgy and council, as a rule, even if they are pressured (by various means and contradictory actions) to not question them.
If the new-rome rule was a strict acceptance, then the indult communities (and the sspx) would've been excommunicated (with no dialogue, or discussions) long, long ago. But the indult communities remain.