The OP question cannot be answered because it makes no sense. I wouldn't be sedevacantist if I thought there were a real possibility that it were wrong. It's like asking a Catholic, "What would you do if Scientology were proved to be true?" If I could come up with a serious answer to that question, I would not really be Catholic in the first place.
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In any case, with sedevacantism, most people arrive at that idea more because of the lack of any alternative explanation for what has happened since the death of Pius XII. A real pope can't proclaim heresy at Vatican 2, can't approve the evil 1983 code of canon law, can't impose the new "Mass" on the Church, can't canonize evil people like Mother Theresa or Paul VI, and can't teach modernism. So what's left? Clearly they are not popes, since the alternative is impossible. And sure enough, even a brief glance at canon law, Church history and the teaching of all the Fathers, theologians and Doctors of the Church confirms what was already obvious, namely that 1) a heretic cannot validly hold office in the Church, and that 2) if a prelate in the Church publicly professes heresy, he automatically loses both his membership in the Church and of course his authority too.
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How about we turn this around? What would you do, XavierSem, if it were proved that the papal claimants since the death of Pius XII have not been valid popes?
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Come to think of it, what would you do if one of your "popes" made a law that people living in unrepentant adultery can receive Holy Communion? Oh wait ...