The funny (strange, not ha ha) thing is that I have plenty of sins in which I stand condemned even by the Conciliar Church. I'll be condemned to hell with or without sedevacantism, barring a miracle of God's grace. Jesus save me, Mary pray for me.
You hit on a very important point in saying this. One my own confessor has put into words a couple of times that I've heard. That is, when we get to heaven, the issue is not going to be nearly so much "who the pope was" as it will be whether we adhered to the Catholic Faith, whether we lived it, whether we held to the ten commandments, the commandments of the Church ,etc..
If a person dies confused about this situation, their minimum commitment, so to speak, to the truth, (as far as I can tell) will have to be the Catholic one: That the Church has a head, a pope, and that the Church cannot "die out" (and therefore cannot need "reviving" or "re-creating" or "rebirth" which implies hell prevailed against it, even if for a time). Anything that is Catholic dogma or known Catholic truth, which we MUST believe to be saved, well... obviously we must believe it. So if God says, there is a pope, and the Church will always exist, then... that is that. If we die confused about WHO the pope was, God may forgive us, depending on various factors like our understanding, or how much of our decision was just wanting to get off easy, or pride, etc.... whether or not we really did what we could to sort it out... whatever.
However... if we get to the judgment seat, and God finds that not only did we go awry on the pope question (which in many cases is understandable), but that we also went awry on all of the virtues and commandments in the process... THEN we've got issues.
I'm not going to say that nobody will go to hell for denying the true pope. The reason I'm not going to say that is, I believe that like the Faith itself, if you really want to know, and you humble yourself enough, and are willing enough to face the trials and difficulties of finding out, then you CAN find the truth, with pretty much full certainty, if not full certainty. The facts, the legal issues, the technicalities... everything will point to the true answer, if only we turn off our pride and our overwhelming desire to condemn others, and just look at the naked (albeit possibly unpleasant) facts.
However I will say that we can ABSOLUTELY and of course, go to hell for abandoning the commandments, or... dare I say it? ... things like charity, humility, etc., which are vital to a true Catholic life.
Don't be discouraged in this hour of darkness. Whatever our understanding, the whole situation is not an easy one. But I think you are on the right page to be as concerned about your personal sins as you are about arguing to the death about this issue.
However, I would say... don't despair, however black the sins. If God can forgive harlots and people who killed Christians, and every manner of grave offense, because of His goodness, mercy and love, we should not doubt that if we are sincere in our sorrow for our sins, that God, our perfect and loving father, will forgive us as well. Sometimes, depending on the gravity of our sins, that can SEEM like a lot of nerve or presumption on our part, but... the reality is, we are speaking of a perfect God, whose virtues are infinitely above our own, and even above our understanding. Now any decent parent whose child has gone awry... however far awry... even to the extent of murder or other crimes that cry to heaven for vengeance... would nevertheless, if they were good, forgive that person if they repented sincerely of their crimes. (To quote loosely: ) But if you, being evil, know how to give good gifts to them that ask thee, how much more will your Father who is in heaven give good gifts to them that ask Him?
If we, being flawed, have mercy upon those many would not have mercy upon, because we know that in the end, a person can truly regret their actions and decide not to do them anymore, so, too, we can be sure that God has such forgiveness, and more, being perfect. Moreover, He knows us better than we know ourselves, and what things prompted us to fall to begin with, and whether or not we knew what we were doing, or meant to do wrong, or what have you. Yet even if we were wholly malicious, the Good God does not want to see us burn in hell, because He is perfect goodness. He would much rather we repent, and spend eternity with Him, than to loose any one of us.
Never give up that fight. In time, God will sort out the question of the pope, and I think history will show in retrospect (at that time) that the SSPX was dead on. In the meantime, arguing about it with people who will never budge is pretty much a waste of time, and an occasion of scandal or deception for ourselves in many cases (especially if we know we're tempted to just give up and throw the baby out with the bath water). But if you adhere to what you know is true, and live the best you can according to God's laws for the love of Him, then I'm sure that He and the Blessed Virgin will never give up on you. God does not give up on us. We walk away from Him.
If you suffer from doubts and strong temptations, and are inclined to despair, I strongly, strongly recommend getting this book and doing the consecration (or renewing it with this book, because it's very much like a retreat):Consecration to Mary
I can't say enough about this devotion, or this book in particular. It is VITAL for souls inclined to doubts and despair to attach themselves firmly to Mary. We may be "naughty little children" in terms of our sins and the poor condition of our spiritual life, and the lack of devotion... but Mary is the Mother Christ gave to such children, so that none of them will be lost. This devotion, I think, is all but indispensable to such souls. (I know, because I'm one of them!)
I also cannot say enough about the merits of making a general confession. It may be a difficult thing to drag one's self through in a sense, because obviously, we're deeply ashamed of what we've done through our whole lives, but... there is nothing like the peace of walking out of that confessional knowing that at that moment, the devil has absolutely NOTHING on you. It's a great blow to feelings of despair in one's salvation!
Anyhow, it's nothing the priest hasn't heard before. However bad we are, or whatever horrible things we may have done, someone else has always done something worse.