It has been said that no one may judge a pope because that is a thing proper to a superior, and he has no superior. But try this reasoning. God is his superior, and when a pope is in formal heresy he is judged by God who with divine law places the pope outside of the church, at the same time he ceases to be head of the church.
Correct. When a man who claims to hold ecclesiastical office, including, and especially the papacy, teaches heresy and or engages in heretical acts, he shows himself to be judged already, by God. Thence the man whose exterior words and actions we judge is not our judging the "pope" but our judging a man separated from the Catholic Church through heresy plain and simple. A purported pope who becomes a heretic, loses his office (if he ever held it to begin with) by that very fact without any need for a declaration, though a declaration that declares what already has happened could be useful.
The sin, culpability is judged by God, this pertains to his salvation or damnation.
The exterior act(s) is judged by faithful seeking to preserve their souls from error, this pertains to his ability to legitimately hold ecclesiastical office.
A private formal heretic (he does not teach heresy or engage in heretical acts and therefore does not destroy the visibility and unity of the Church) can be a valid Pope that goes to Hell.
A public material heretic "Pope" cannot be Pope (because a Pope is prevented by the Holy Ghost from destroying the visibility and unity of the Church) but could go to Heaven if he embraces heresy through no fault of his own. But one who claims ecclesiastical office, it would seem, does not have ignorance as an excuse. All the V2 Popes until now have been born and raised in the true Church and were aware of what the Catholic Church has always taught. Eventually, if God allows this to continue, we may have a "Pope" who really is ignorant of his heresy through no fault of his own, but if he teaches heresy, after being shown his error once or twice and pertinaciously holds to his heresy, he will not be a valid Pope according to Divine Law.
The "Popes" since 1958 have be formal public apostate heretics of the worst kind imaginable. These, should they die in this state, appear destined to Hell barring some last minute miraculous conversion.