“However, the fathers and doctors of the Church all agree that IF a Pope ever were a manifest heretic, he would cease to be a valid Pope, for one cannot be the head of a Church of which one is not a member, and heretics are automatically excommunicated.”

Ahem, no, they very obviously do NOT agree, lest we pretend Bellarmine, Cajetan, JST, Billot, Billuart, Suarez, Vitoria, et al. were all on the same page (with most opposing the quoted statement)?

Finally, the Holy Fathers teach unanimously not only that heretics are outside of the Church, but also that they are "ipso facto" deprived of all ecclesiastical jurisdiction and dignity. St. Cyprian (lib. 2, epist. 6) says: 'We affirm that absolutely no heretic or schismatic has any power or right'; and he also teaches (lib. 2, epist. 1) that the heretics who return to the Church must be received as laymen, even though they have been formerly priests or bishops in the Church. St. Optatus (lib. 1 cont. Parmen.) teaches that heretics and schismatics cannot have the keys of the kingdom of heaven, nor bind nor loose. St. Ambrose (lib. 1 de poenit., ca. 2), St. Augustine (in Enchir., cap 65), St. Jerome (lib. cont. Lucifer.) teach the same.

Chapter 21: From the vision of Revelation chs.17 and following the new error about Antichrist is refuted rather than confirmed.
[...]
8. In whatever way, then, that prophecy be understood, therefrom can it not only not be collected that Rome is already now antichristian, but also neither can it with any likelihood be inferred that the Church, which is now at Rome, either is already, or will at some time be, antichristian and infidel, because although many citizens and members of it should defect from the faith, they come to be by that very fact outside the Church, which always remains faithful. And much less does it follow that the head of the true Church is or will ever be Antichrist, because although the city is thoroughly destroyed because of apostasy, or for any other cause whatever, the head and rock of the Church could stand firm, or reside elsewhere, or go into hiding. Nay, although God should permit the person of some Pontiff in those times to fall and to yield to the tyrants by professing errors, not for that reason would the Pontiff, as he is Pontiff, fall, nor would the Church fall, but, after deposing him, the Church could for itself create a Pontiff. Which is said for exaggeration and for more explaining the thing; for it is more credible that God will not permit it, especially during so great a tribulation and oppression of the Church.