You don't think it's something akin to a dogma that a public, manifest heretic can't be Pope? While it may not have been explicitly defined, isn't it Pharisacal to brush it aside on that basis? It's more like common sense than like a dogma, I'll grant you. So maybe the SSPX is not so much heretical as just irrational; and it really is.
Because if a manifest heretic who teaches soul-damning error to the universal Church can be Pope, the Church makes no sense; it's like saying that God can lie. How would we ever know the truth in those circuмstances? The only way would be to appoint a de facto Pope to sift the sayings of the nominal Pope. That is what SSPX has done, the head of the SSPX is in reality, for those who are in SSPX, a de facto Pope, who is looked to the way that Catholics should only look to the Pope. Because if he can tell us when to listen to the Pope and when not to, if he has veto power over the Pope, what does that make him? That's right, kids -- bigger than the Pope, more powerful than the Pope, ergo, the de facto Pope.
Then there are others in SSPX who just deny that they are heretics, as you go on to say ( excellent post by the way ). That is even more irrational.
A priori, I think most of us accept the theological possiblity of the thing itself: But there are some who contend the "popes" have not done such and such, but they actually make the point; for their argument hinges on the same proposition as ours: That he did NOT DO what we accuse him of doing.
And to try to prove their point, they have to deny the evidence of their own eyes. It reminds me of that line from A Day at the Races with Groucho Marx. He is dressed up as a doctor, pretending to be a doctor, but with bare hairy legs sticking out of his lab coat, looking like a madman, etc. Someone comes in and isn't quite sure they trust him, for obvious reasons. Groucho keeps telling him he's a doctor, and when it's still not working, he says "Who are you going to believe, me or your own eyes?" Genius line that so accurately describes the superstitious, hoaxed, scared-of-their-own-shadow masses: "Oooh, we might go to hell if we say the obvious non-Catholic who they show on TV is not a Pope." Even if you were WRONG, God does not work like that; if you are sincerely wrong, you wouldn't go to hell for being a sede. I said it before and I'll keep saying it, despite our age of technology and the information superhighway, we are more superstitious and less able to think clearly than illiterate medieval peasants with common sense.