as long as he is not declared or judged to have legitimately been deprived of his rule, is always the supreme judge...
Just little bit of reading comprehension goes a long way. He is judged to HAVE been deprived of his rule. They're judging something that had already taken place.
No, St. Robert Bellarmine did not hold Cajetan's opinion

... unless he was a total idiot and somehow didn't realize that Cajetan held the same opinion he did.
St. Robert cited the case of Pope Celestine's declaration regarding Nestorius, that Nestorius had lost his authority from the time he began preaching heresy, several years before he was officially / materially deposed.
Essentially, St. Robert was a sedeprivationist or sedeimpoundist before the terms existed, acknowledging two separate aspects of office, the formal which is stripped by God the moment one becomes a manfiest heretic, and the office itself which can be stripped later. In fact, discussion of the material aspects of the office and the formal originate in the thinking of St. Robert.
Unfortunately, small minds that are incapable of grasping these distinctions somehow try to pretend, laughably, that Bellarmine held Cajetan's opinion.