Have heretical anti-popes invalidly reigned for decades?

One of the dangers of sedevacantism is that it can become a kind of universal acid that eats through the entire papacy, not just recent modern popes. For example, sedevacantists who focus on moral scandals among cardinals and the curia in the last forty years as evidence that the papacy is vacant would also disqualify several medieval popes, especially those who reigned during the infamous pornocracy of the ninth and tenth centuries. This time period included the infamous cadaver synod of Pope Stephen VI, where he put the corpse of his predecessor Pope Formosus on trial. If they focus on theological errors that allegedly invalidated past popes, then they are going to end up no different than Protestants who try to find similar errors in all the popes, not just modern ones. They would end up invalidating popes like Honorius, whom a later ecumenical council (Constantinople III) sought to condemn as a heretic for failing to stop the Monothelite heresy, although Pope Leo II, as head of the council, did not confirm the condemnation. By creating enough gaps in papal succession through these arguments, all of apostolic succession would be threatened. The examples can be multiplied but, once again, my big concern is that the logic of sedevacantism and its desire to unseat recent popes could be weaponized against the entire papacy.

Sedevacantism is basically Protestantism dressed up in a Catholic costume. It claims to be faithful to Tradition, but how can that be when prominent sedevacantists say that the faithful just have to stay home on Sunday and pray the rosary? How much more unfaithful can you be to Catholic Tradition than by saying that the Holy Eucharist, along with Christ’s visible Church, has disappeared from the face of the earth? At this point, there is no path for Christ’s Church to be restored from this alleged apostasy, which is why many of them say that we must be near Christ’s Second Coming and the end of the world. But what’s more likely: that this apocalyptic sect correctly discerns the end times even though Christ said that no man knows that, or that they are simply leading people away from the Church which has faithfully persevered for two thousand years?