More and more, like Bellarmine, I lean towards believing that a true pope could never fall into heresy as pope.
I think a lot of SVs have moved to that position, believing instead that these guys were never popes in the first place.
To me, though, if they were not manifest heretics before their election, then there must be some other reason for their illegitimacy, i.e. an illegitimate election. To me that's yet another point in favor of the Siri Theory. With that theory, Roncalli, Montini, and Wojtyla were never legitimately elected in the first place.
I feel that if a Pope like Roncalli hadn't been a manifest heretic before his election (he was under suspicion at the Vatican, but that's a step removed from being a manifest heretic), that means he was effectively an occult heretic and so would have been legitimately elected pope (unless you hold to the extreme theory that even occult heretics cannot be popes ... forget which # that is). I believe, again just a "pious belief", that an occult heretic who became a legitimate Pope would be converted, but as certainty that he would be prevented by God from wrecking the Church, even if it meant that God would strike him dead before he could do so.
And that's another intriguing aspect of Siri Theory, that the conspirators waited until Siri had accepted (according to Paul Williams, he took the name Gregory XVII, which would not have happened, nor would the white smoke have happened, until he accepted). I think these conspirators realized also that a legitimate Pope would be protected by the Holy Ghost from wrecking the Church. They thought they had their man in Pius IX, but he converted from his liberalism (possible Masonic membership also) and turned on them, to the point that the Masonic lodges went through the trouble of "excommunicating" him from Masonry ... assuming it was true). Pius IX had been perhaps THE most liberal Cardinal in the Church before his election. In any case, they waited until Siri was elected and accepted, where they could more easily have just threatened him before the conclave, since Siri had been considered the undisputed favorite "papabile" before the election, and the stated choice of Pius XII. But if they had threatened him from accepting, then even though it was under duress and threat, there's no provision for that in law. He wouldn't have been the legitimate pope. But once he accepted, the law holds that any resignation under duress or threat is considered null and void. Thus the replacement candidate would have been illegitimate, and therefore free to perpetrate his wreckage upon the Church.