Now then, going back to the parallels between the 4th century and today, if the invalidity doesn't kick in until after a formal "condemnatory or declaratory judgment" (assuming JAM is correct, which I'm not challenging), there seems to be a problem if we have a situation in which the pope is a heretic and hence not pope, and furthermore unwilling to condemn the very same heresy he upholds. Liberius, who excommunicated Athanasius, may have been such a one, and if the sedevacantists are right, we have the same situation with the Conciliar popes. Such a "pope" would let heresy slide, and even if he condemned someone for anything it wouldn't count because he really wasn't pope, at least in the thinking of many on this forum, and very possibly in actual fact. I say possibly because the Church is headless right now, either without a real pope, or at least without one who is doing his job, so there is no one in a position of authority to deal with this crisis--or if there is, he's awfully hard to find True, there are records of earlier de fide pronouncements, etc., but the interpretation of them is hotly contested. To a large extent we're on our own.
The biggest difference I see between the 4th century and now is the change to the Mass and sacraments, but despite the NO being flawed and easily abused, it's not crystal clear (intellectually as opposed to emotionally) that a properly said NO is so evil that the real Church can't possibly have promulgated it. Nor is it crystal clear that any or all the sacraments have all been ruined, despite problems with them.