A Catholic is not thereby absolved from the duty of obeying the Pope, but the Pope's authority only exists within a limited sphere (like anyone else's), and no superior has the authority to command a subordinate to sin.
More concretely, if the Pope expresses a heretical opinion, I am not bound by any duty to agree with him. (Him expressing an opinion is not the same as him commanding obedient agreement.)
If he expressed a heretical opinion and then attempted to compel me to agree with his error, I would be required by my conscience to respectfully refuse. Refusing obedience in a specific case is not the same as deposing him.
(By the way, if Donald Trump attempted to compel me to go against my conscience and commit a sin, exactly the same response would be appropriate. However, he would remain the President just as Francis remains the Pope.)
2) a non-Catholic who has left the Church by reason of failure to submit to the Pope?
I think there's a third option:
3) a faithful Catholic dissenting with a Papal view which is not de fide. That is to say, I am not obliged to submit to the Pope except when he is either (a) speaking ex cathedra, or b) repeating something that has always been part of the Deposit of Faith (Public Revelation, which has not changed since Apostolic times)
There's likely to be debate about that; but I reckon I can't be excommunicated without canonical due process. That's how I understand it.
Now that brings up an interesting question: could this work the same was a Constitutional test case would work in U.S.law? In other words, if I were excommunicated, could I use appeals to require more and more explanations of the charges and cross-examination of accusers, etc?
That would sure be interesting.
I don't know.. I keep saying "I'm no canon lawyer, thank God."
I know several Catholics who are now canonized saints historically had been excommunicated unjustly and then restored. Two names pop into mind: St.Hildegard of Bingen, and St. Mary MacKillop of Australia. I know they were restored to communion when they proved their innocence, but I don't know the details of how that happened.
Calling all FReeper canon lawyers! Anybody know?