In a chapter, “On the utility or even the necessity of celebrating Councils,” Bellarmine wrote the following which would tie into his discussion of a possibility of a heretical pope as well as the Sedevacantist debate within traditional Catholic circles:
d) The fourth reason
is suspicion of heresy in the Roman Pontiff, if perhaps it might happen, or if
he were an incorrigible tyrant; for then a general Council ought to be gathered
either to depose the Pope if he should be found to be a heretic, or certainly
to admonish him, if he seemed incorrigible in morals. As it is related in the 8th
Council, act. ult. can. 21, general Councils ought to impose judgment on
controversies arising in regard to the Roman Pontiff—albeit not rashly. For
this reason we read that the Council of Sinvessano in the case of St.
Marcellinus, as well as Roman Councils in the cases of Pope Damasus, Sixtus
III, and Symmachus, as well as Leo III and IV, none of whom were condemned by a
Council; Marcellinus enjoined penance upon himself in the presence of the
Council, and the rest purged themselves (See Platina and the volumes of
Councils).
e) The fifth reason
is doubt about the election of a Roman Pontiff. For if the cardinals could not
or would not create a Pope, or certainly if they all died at the same time, or
a true doubt should arise for another reason to whom an election of this sort
would pertain, would look to a general Council to discern in regard to the election
of a future Pope, although it does not seem to be realistic to expect this
would ever happen. (Robert Bellarmine, De Controversiis: Tomus II On the
Church, Volume 1: On Councils, on the Church Militant, On the Marks of the
Church [trans. Ryan Grant; Post Falls, Idaho: Mediatrix Press, 2017], Book
1 Chapter IX, p. 40)