But what happens to the Siri thesis beginning with Popes elected after Cardinal Siri died?
John XXIII was not a formal heretic before the election, so the Siri thesis says that his election was null and Siri was the real Pope.
Not quite. Siri thesis has to do with the election itself, not the imputation of heresy to anyone. So, the way it goes, Siri was elected, accepted the papacy, even picked the name Gregory XVII. THEN the infiltrator Cardinals threaten him somehow to step down, and he complies. That resignation would be invalid since it happened under duress. At that point they install Roncalli, who's illegitimate not because of any heresy, but because Siri was still the rightful pope.
Interestingly, there's a prophecy from St. Francis that refers to an "uncanonically elected pope" that would become a destroyer.
So, this makes a lot of sense. I agree with Gary Giuffre that they deliberately engineered it this way because a validly elected pope would be prevented by the Holy Ghost from wrecking the Church ... where God would strike him down or else convert him. Pius IX was arguably THE MOST LIBERAL Cardinal in the Church when he was elected ... and there are some not-altogether-unsubstantiated allegations that he had actually become a Freemason ... but then God converted him into a strong anti-Modernist Pope. But God's protection over the Papacy would not apply to an Antipope ... which is why they waited for Siri to accept. They could just as easily have contacted him BEFORE the conclave, threatened him then to just not accept. There's no notion there that refusal to accept under duress makes you the rightful Pope.
So the most common SV explanation, that Montini never became pope due to manifest heresy simply fails ... since there's no evidence of manifest heresy before his election. Father Cekada, in addressing Salza's book, stated that nearly all SVs have abandoned the argument that Montini FELL from the papacy, but hold that he never had it in the first place. OK, well, where's evidence for manifest heresy before Montini's election. I've never seen anything. Now, people claim he ratted out clergy behind the Iron Curtain and was a sodomite ... but neither of those are heresy where he'd cease to be a Catholic, and they're not proven at this time, just various rumors out there.