If he is right, the man on the chair, prior to the declarative sentence, is still a true pope. Impound him - whatever - he is still issuing, or has issued, false and/o evil teachings and laws while in the seat.

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No human being has any jurisdiction, whether spiritual or temporal, over a true pope. That means no one can pass sentence against a true pope.
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This is why the Fathers of the Church don't teach any such thing. They all teach (and St. Robert Bellarmine says it is the unanimous teaching of the Fathers) that a pope who taught heresy would fall from office by that very act, and the cardinals would then make a declaration of the fact that he had already lost the papacy and then proceed to elect a pope. But obviously this is only possible if the former-pope had already lost his office before the cardinals made a declaration to that effect and then elected a replacement.

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No human being has any jurisdiction, whether spiritual or temporal, over a true pope. That means no one can pass sentence against a true pope.
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This is why the Fathers of the Church don't teach any such thing. They all teach (and St. Robert Bellarmine says it is the unanimous teaching of the Fathers) that a pope who taught heresy would fall from office by that very act, and the cardinals would then make a declaration of the fact that he had already lost the papacy and then proceed to elect a pope. But obviously this is only possible if the former-pope had already lost his office before the cardinals made a declaration to that effect and then elected a replacement.

Fr. Chazal's heretic pope is still pope until the declarative sentence. He is still "true" pope in that sense. That's all that is meant. 

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I'm not sure I follow your point, but no sentence of any kind can be passed against a true pope.

Just call it a declaration then. 

All acknowledge that the Church can declare that God has deposed the pope.

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Yes, that is my understanding too. It is not human beings who depose a pope, but they can only acknowledge that he has lost his office already, or, as you put it, been deposed by God.

I think the three of us agree, just like in Belloc's Christmas carol:
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May all good fellows that here agree
Drink Audit Ale in heaven with me.
And may all my enemies go to hell
Noel! Noel! Noel! Noel!
May all my enemies go to hell
Noel! Noel!

Not a single one of the Church Fathers that Bellarmine quoted or referenced in support of his position said any such thing.  In fact, none of them taught that a manifest heretic is ipso facto deposed.  Not a single one.

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Here's what St. Robert Bellarmine said:
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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”. 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, those who are cast out as excommunicates, or leave on their own and oppose the Church are separated from it, namely heretics and schismatics.

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Here's what St. Robert Bellarmine said:

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”. 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, those who are cast out as excommunicates, or leave on their own and oppose the Church are separated from it, namely heretics and schismatics.

I know what he said, but what I am saying is that not a single one of the Fathers of the Church that Bellarmine quoted - not St. Cyprian, who he quoted three times, not St. Jerome, who he quote once, and not St. Athanasius, St. Augustine, St. Ambrose or Optatus, who he referenced - taught that manifest heretics are ipso facto deposed.  Not one.  

In the quotation you provided above, Bellarmine quotes St. Cyprians commentary on Novation to support his position.  Let's see what Cyprian actually taught.

To begin with, Novation never held office in the Church or possessed jurisdiction.  He left the Church after Cornelius was elected Pope, because he considered him to be too liberal, and was illicitly consecrated outside the Church by three bishop he had previously left the Church.   Soon thereafter, Novation declared that he was the true Pope.  

Now, in the quotation Bellarmine references above, St. Cyprian says is even if Novation had been legitimately consecrated a bishop before he separated from the Church, he would not be have been able to remain a member of the episcopate, since he separated from the Church and from unity with the other bishops.   Here is the entire quote from St. Cyprian, with the that portion Bellarmine quoted underlined:

St. Cyprian: “In reference, however, to the character of Novatian, dearest brother, of whom you desired that intelligence should be written you what heresy he had introduced; know that, in the first place, we ought not even to be inquisitive as to what he teaches, so long as he teaches out of the pale of unity. … schismatics are always fervid at the beginning, but that they cannot increase nor add to what they have unlawfully begun, but quickly fail along with their evil emulation. But he could not hold the episcopate, even if he had been made a bishop before he departed, since he has cut himself off from the body of his fellow bishops and from the unity of the Church; since the apostle admonishes that we should mutually sustain one another, and not withdraw from the unity which God has appointed, and says, ‘Bearing with one another in love, endeavoring to keep the unity of the Spirit in the bond of peace’ (Ephesians 4:2-3).  He who observes neither unity of spirit nor the concord of peace, and severs himself from the bonds of the Church, and from the fellowship of her priests, cannot have episcopal power or honor, since he has refused to maintain either the unity or the peace of the episcopate. (Bk 4, Epist. 2).

It was because Novation separated himself from the Church and from the body of bishops that he would have been unable to retain episcopal power or dignity, not merely because he fell into personal heresy.  It is also worth noting that Cyprian never says Novation would have lost his jurisdiction ipso facto, without having to be formally deposed by the Church.  All he says is if he would not have been able to retain the episcopal power and dignity if he failed to remain in union with the episcopate.

The other authority Bellarmine quoted above is John Dreido, and he doesn't help his opinion either.   All Driedo says is some Christians are cut off by the Church's judgment, while others depart from the Church on their own. No great revelation there. Here is the actual quotation:

John Dreido: “All those who have received the sacrament of faith [Baptism], and are visibly attached to the Church, and associate in a peaceable way with the Christian people, are in the Church until they are either cut off by the Church’s judgment, or depart of their own accord."

Nothing there about a manifest heretic being ipso facto deposed.

The other quotations Bellarmine references in the chapter (in response to the fourth opinion), the ones from St. Augustine, St. Athanasius, St. Jerome, the other two from St. Cyprian, and the one from Optatus, don't support his position that "a manifest heretic is ipso facto deposed" either.  In fact, none of those quotations even mention a bishop losing his jurisdiction.  The quotation from Pope Celestine regarding Nestorius was also a fail.  All Celestine said was the excommunications Nestorius pronounced against those who refused to accept his heresy was null and void.  What Bellarmine doesn't mention, is that Pope Cornelius said Novation remained in communion with himself and with the Church even after he had been formally judged a heretic, and had been warned three times to renounce his heresy. Nestorius retained his office until he was deposed by the Council of Ephesus, in July of 431 - which was 11 months after he had been judged a heretic by the Pope himself at a Council in Rome.

To say Bellarmine failed to prove his position would be an understatement.   What Bellarmine did proved, is that he was not aware of a single Father of the Church, or any theologian, who supported his position, since if he had known of any, you can be sure he would have quoted them.