Right. But he argues that if a "pope" were to fall into heresy, he would ipso facto fall from the papacy and the Church. That is the Bellarmine opinion we are addressing.

Yes.  Thank you for the clarification.  The condition for Opinion No. 5 (that a pope can become a public manifest formal heretic) is purely hypothetical.

I respond: there are five opinions on this matter.

The first is of Albert Pighius, who contends that the Pope cannot be a heretic, and hence would not be deposed in any case [319]: such an opinion is probable, and can easily be defended, as we will show in its proper place. Still, because it is not certain, and the common opinion is to the contrary, it will be worthwhile to see what the response should be if the Pope could be a heretic.
...

Now the fifth true opinion, is that a Pope who is a manifest heretic, ceases in himself to be Pope and head, just as he ceases in himself to be a Christian and member of the body of the Church: whereby, he can be judged and punished by the Church. This is the opinion of all the ancient Fathers, who teach that manifest heretics soon [mox — better translation: immediately] lose all jurisdiction, and namely St. Cyprian who speaks on Novation, who was a Pope in schism with Cornelius: “He cannot hold the Episcopacy, although he was a bishop first, he fell from the body of his fellow bishops and from the unity of the Church” [332]. There he means that Novation, even if he was a true and legitimate Pope; still would have fallen from the pontificate by himself, if he separated himself from the Church. The same is the opinion of the learned men of our age, as John Driedo teaches [333], those who are cast out as excommunicates, or leave on their own and oppose the Church are separated from it, namely heretics and schismatics. He adds in the same work [334], that no spiritual power remains in them, who have departed from the Church, over those who are in the Church. Melchior Cano teaches the same thing, when he says that heretics are not part of the Church, nor members [335], and he adds in the last Chapter, 12th argument, that someone cannot even be informed in thought, that he should be head and Pope, who is not a member nor a part, and he teaches the same thing in eloquent words, that secret heretics are still in the Church and are parts and members, and that a secretly heretical Pope is still Pope. Others teach the same, whom we cite in Book 1 of de Ecclesia. The foundation of this opinion is that a manifest heretic, is in no way a member of the Church; that is, neither in spirit nor in body, or by internal union nor external. For even wicked Catholics are united and are members, in spirit through faith and in body through the confession of faith, and the participation of the visible Sacraments. Secret heretics are united and are members, but only by an external union: just as on the other hand, good Catechumens are in the Church only by an internal union but not an external one. Manifest heretics by no union, as has been proved.

We need to get this correct guys. Bellarmine is a Saint. We can't misrepresent him. As you can see below, Bellarmine held the 5th opinion, although he did not reject the 1st opinion completely.

After introducing #1, St. Robert says that "it will be worthwhile to see what the response should be if the Pope could be a heretic".  So he's discussing #2 - #5 within the constraint or context of what the "response should be" IF #1 is not the case.

Now the fifth true opinion, is that a Pope who is a manifest heretic, ceases in himself to be Pope and head, just as he ceases in himself to be a Christian and member of the body of the Church: whereby, he can be judged and punished by the Church. This is the opinion of all the ancient Fathers, who teach that manifest heretics soon [mox — better translation: immediately] lose all jurisdiction, and namely St. Cyprian who speaks on Novation, who was a Pope in schism with Cornelius: “He cannot hold the Episcopacy, although he was a bishop first, he fell from the body of his fellow bishops and from the unity of the Church” [332]. There he means that Novation, even if he was a true and legitimate Pope; still would have fallen from the pontificate by himself, if he separated himself from the Church. The same is the opinion of the learned men of our age, as John Driedo teaches [333], those who are cast out as excommunicates, or leave on their own and oppose the Church are separated from it, namely heretics and schismatics. He adds in the same work [334], that no spiritual power remains in them, who have departed from the Church, over those who are in the Church. Melchior Cano teaches the same thing, when he says that heretics are not part of the Church, nor members [335], and he adds in the last Chapter, 12th argument, that someone cannot even be informed in thought, that he should be head and Pope, who is not a member nor a part, and he teaches the same thing in eloquent words, that secret heretics are still in the Church and are parts and members, and that a secretly heretical Pope is still Pope. Others teach the same, whom we cite in Book 1 of de Ecclesia. The foundation of this opinion is that a manifest heretic, is in no way a member of the Church; that is, neither in spirit nor in body, or by internal union nor external. For even wicked Catholics are united and are members, in spirit through faith and in body through the confession of faith, and the participation of the visible Sacraments. Secret heretics are united and are members, but only by an external union: just as on the other hand, good Catechumens are in the Church only by an internal union but not an external one. Manifest heretics by no union, as has been proved.

Bottom line is that since Vatican I only Bellarmine's opinion remains standing.  It is absolutely essential that the Pope have been ipso facto deposed by God BEFORE the Church could render any kind of judgment regarding the man who used to be Pope.  To hold anything else would be to assert the heresy condemned by Vatican I that the Church can pass judgment on a Pope.

PS to DR: Does 6 theologians suffice?  If not, there's more at the link just provided.