Q: St. Robert Bellarmine only gives his opinion on a heretical pope!
It is very hard to ignore St. Bellarmine and his works – “The Controversis on the Marks of the Church” and “The Controversis on the Roman Pontiff” (English translations by Ryan Grant were made available this year.) In the Translator’s Preface we read:
“Among the Theologians, Bellarmine was perhaps the most prolific and no one is more quoted in the documents of Vatican I (…). He not only refutes Protestant teaching, but lays down the theological foundation which would make their way into the definitions of the First Vatican Council.”
Moral Theology (Moral Theology: A complete course based on St. Thomas Aquinas and the best modern authorities; John McHugh, O.P., Charles Callan, O.P., N.Y., 1929; Vol. II) also teaches:
“If the opinion has the support as certain of only one theologian, it may be followed without further investigation, if he has received special mention from the Church as an authority and a safe guide.”
St. Bellarmine did receive special mention from the Church as an authority and a sage guide on matters of the Papacy by Pope XI in 1931, as he was appointed Doctor of the Church:
“Moreover he appeared even up to our times as a defender of the Roman Pontiff of such authority that the Fathers of the  Vatican Council employed his writings and opinions to the greatest possible extent.
Additionally St. Bellarmine wasn’t the only one who held the opinion that “ a manifest heretic would be ipso facto deposed, is proven from authority and reason”; we can add the authority of two Popes – Innocent III and Paul IV, who in their times had a fear of a heretic being elected to the Papacy.