I do not necessarily disagree with you on the points you made, the thing you are missing is that he did not give anyone permission, nor did he make anyone responsible for declaring the pope was not the pope or that he was never elected.
For whatever reason, sedevacantists believe it is their duty to declare the pope is not the pope and that he was never elected, but this is something they do not have any authority to do, which makes the whole idea meaningless, a "tinkling symbol", a colossal waste of time because there is nothing anyone can actually do about it even if true, and the sedevacantist opinion goes against the dogma that we must be subject to him, therefore sedevacantists needlessly risk their salvation.
I said they needlessly risk salvation because one can remain a subject to even the worst modernist heretic of a pope imaginable, and still get to heaven. But per the dogma, one cannot get to heaven without being subject to him - the odds of sedevacantists being wrong are too high, even if there's only one chance in a billion that the sedevacantists are wrong, that's too high and needlessly risks your eternity on an opinion.
According to cuм ex, the only thing anyone can actually do about a heretic pope is "contradict him". This, according to cuм ex, anyone can do because he did not specifically appoint anyone in particular - least ways not according to the translations I've seen.
From an earlier post:
Would you like to know what contradicting a heretical pope means? Below is the best definition of contradicting a heretical pope is that I know of. In my opinion, THIS is what Pope Paul IV meant. This is the Catholic reaction to the situation.
Also, although it might have changed nothing, consider what the situation could be today if this would have actually happened at/after V2, or even today - from Who Shall Ascend?.........
However, even though the hierarchy cannot take legal action against an heretical pope, all of them together, or any one of them in particular, can condemn his teaching; they can accuse him before God's tribunal, warn him of his sins, and remind him of the divine wrath. Should this measure fail to produce any correction, they can denounce him before his subjects, the Catholic faithful, and warn them that they are not to listen to his teaching. Indeed, not only may the prelates of the Church do this, they have a most serious obligation to do it, an obligation which is as grave as the heresies are pernicious and scandalous. And if they fail to do this, they become a party to the pope's crimes, and will most certainly share in his punishment.
Moreover, where the bishops default in their solemn duty to protect the Church and God's Little Sheep, the priests and the laypeople have not the right, but the duty, to raise their voices against an heretical pontiff. They not only raise their voices to God in prayer for the misguided man, but they also speak out to the bishops and the priests, and among themselves so as to warn their brothers and sisters in Christ that the plague of heresy has infected even their Holy Father, and has rendered him dangerous and unclean.