I do wish the debates would focus less on the "heretic pope" question and more on the broader questions of ...
Is the Conciliar Church the Catholic Church?
and
Is it possilble for a legitimate Pope to destroy the Church, pervert the Magisterium, institute a (bad imitation of a) Prot liturgy as the Church's public worship, canonize bogus saints, etc.?
Cassman actually set that up as a softball in his earlier remarks, where he claimed that the Papacy is there precisely to be the rock on which the Chuch is founded and to prevent all these evils to befall the Church. Yes, indeed, and that's why we say these men can't be popes.
You can argue until the cows come home about Bellarmine vs. Cajetan / John of St. Thomas (Bellarmine's opinion is much stronger in that any alternative entails judging a pope and having the declaration serve as a cause of the deposition, and those views are both heretical).