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Sure, but what about after Siri's death?

You say that there's not manifest heresy from Roncalli or Montini. Ok, but what about Ratzinger and Bergoglio? How do you explain Ratinzer's election and "papacy", since it happened after Siri died?

Not sure about Wojtyla.  I know nothing about him before his election.  He was widely reputed to have been a Communist sympathizer, but I know of no public statemens of his that would have indicated maifest heresy.  For Ratzinger, there's volumes of material.

But the really problem, since he was behind V2 was Montini.

From the Siri Thesis perspective, since he didn't die until 1989, the elections of Roncalli, Montini, Luciani, and Wojtyla were invalid.  Ratzinger and Bergoglio came after Siri had died, but for them one might argue that by then there were no or few legitimate Cardinals left, and that Ratzinger wasn't a valid bishop ... for example.

I had not thought about that. Doubtful episcopacy solves the problem for Ratzinger and sucessors in this case.

All the same, I understand that the Cassiciacuм thesis says that the modernist cardinals are legitimate to some extent, and that they could, in theory, elect a valid Pope.

Right.  So, the implications of a doubtful episcopacy are interesting.  One must be a cleric at least to undertake certain aspects or functions of the papacy, e.g. start making appointments, etc.  Ratzinger certainly was a valid priest (therefore a cleric).  But in order to exercise teaching authority, he needed to become a member of the Ecclesia Docens and one must be a bishop to do so.  So he couldn't exercise teaching authority.

So, yes, that's one possible explanation, the other being that by the time he was "elected" there were no legitimate/valid Cardinals left.  This would be true for both straight sedevacantists and for Siri Theorists, since for both those groups these papal claimants weren't even "material popes, whereas for the sedeprivationists the Cardinals could still be valid ... in the sense of having the ability to elect.

Until the Church intervenes, we'll never know exactly what happened, and I think too much time is wasted on arguing about these details.  I think that what SVs, SPs, and Siri Theorists have in common is that we rejection the notion that the Conciliar Church is the Catholic Church.  Rest of it are just attempts to explain, then, how this Conciliar Church was able to somehow eclipse or displace the Catholic Church.  And there's a tendency to conflate this final conclusion with one's favorite explanation, and because the conclusion is dogmatically certain, attempting to assert that the explanation is also dogmatically certain.  That's a logical fallacy.

I've descried myself as a dogmatic indefectibilist.  As far as I'm concerned, any theory that upholds the indefectibility of the Church's Magisterium and Universal Discipline is fair game, and I'm not going to argue too much about it.  If one wanted to believe that the real Paul VI was replaced by a big-eared crooked-nosed double, drugged, and held captive in a dungeon, while the imposter who replaced him wreaked all the havoc, well, more power to you.  I might not buy it ... but you're NOT rejecting the overall indefectibility of the Church's Magisterium and Universal Discipline, which would be preserved under such a hypothesis.

Additionally, in the case of Ratzinger, one could follow the +Vigano argument and say that Ratzinger's “intent to be an orthodox pope” is HIGHLY doubtful at the time of his election (and also afterwards).  If one looks at his history of supporting V2 and the new mass and all the heretical books he wrote before his papacy…the guy was not orthodox.  His intent was to “hybridize” the Trads with the modernists.  Which he was highly successful in accomplishing.  Therefore, his papacy was suspect, from the very beginning, for lack of an intent to be a proper pope. 

This goes double for Francis.  Take 30min and research his abominable, new age, ecuмenical acts in Argentina.  The guy is a flaming modernist.