I privately hold the sedevacantist position, but I don't believe that sedevacantists are the Church. I am reminded of the definition of the Church being all baptized persons who follow the teachings, traditions and Sacraments passed on by the Apostles through the Popes. In other words, I personally believe that the Church is made up of all the various traditional Catholic groups combined throughout the world who follow the True Faith and Sacraments.
I think that a lot of people tend to get wrapped up a bit too much in trying to make the decision of whether the current popes are popes or aren't popes...
Can't everyone just agree that we can't follow them or listen to them and leave the rest to be officially decided later? :)
I referred you not long ago to the second-to-last chapter of "Liberalism is a Sin", but I really think you need to read it again carefully in light of what I am going to say now...
Just because one is a "card-carrying Catholic", so to speak, does not make what he personally believes is automatically OK, nor is it some kind of "immunity-from-criticism" card. A Catholic, though technically still in the Church, can hold dangerous beliefs that harm others. In proportion to the danger and influence, we should condemn their error, even up the point of calling them heretics and keeping away from them. This is what was done in Catholic history. The day before Fr. Martin Luther was excommunicated, he was a Catholic. Yet, it was appropriate to shun him for the danger he was. The same with the Arian clergy before they were officially condemned by the Church. The Arians, regardless of whether they had the true Sacraments and whether an individual priest didn't sermonize about their one error, the direct association with the error was enough to completely shun them and risk bodily harm in the mountains to have the Mass.
'Believing a man is a true pope and totally shunning him and his bishops' is inconsistent in conviction, and its consequences are very seriously damaging to the faith of others. A prime example of this was when St. Paul publicly criticized St. Peter. This was God's working to keep the head of the Church from error before it spread from a local problem to the Church. It a Providential working of the Infallibility of the Church. There is difference of opinion whether St. Peter was guiltless, or whether it was a venial sin, but the point is, when you read the words of St. Paul in Scripture, regardless of intention, the objective serious consequences of failure to act consistently on a belief should be excoriated with strong language to protect others from the same mistake.
St. Peter was having the Jews and Gentiles eat separately. This was at variance with his conviction. Here are the strong words of St. Paul in Galatians 2:11-18 (my emphasis):
"But when Cephas was come to Antioch, I withstood him to the face, because he was to be
blamed. For before that some came from James, he did eat with the Gentiles: but when they were come, he withdrew and separated himself, fearing them who were of the circuмcision. And to his
dissimulation the rest of the Jews consented, so that Barnabas also was led by them into that
dissimulation. But when I saw that they
walked not uprightly unto the truth of the gospel, I said to Cephas before them all: If thou, being a Jew, livest after the manner of the Gentiles, and not as the Jews do, how dost thou compel the Gentiles to live as do the Jews? We by nature are Jews, and not of the Gentiles sinners. But knowing that man is not justified by the works of the law, but by the faith of Jesus Christ; we also believe in Christ Jesus, that we may be justified by the faith of Christ, and not by the works of the law: because by the works of the law no flesh shall be justified. But if while we seek to be justified in Christ, we ourselves also are found sinners; is Christ then the minister of sin? God forbid. For if I build up again the things which I have destroyed, I make myself a
prevaricator."