Sedevacantism is not a Dogma. Catholics who become heretics do not stay Catholic is of divine law.
"Once a Catholic, always a Catholic" is a doctrine of man; that man's name was Fr. Wathen.
You confuse the prot heresy; "once saved always saved", which is a doctrine of man, with the Catholic truth; "once a Catholic always a Catholic".
At some point, you must be honest with yourself and admit that sedevacantism is not a teaching of the Catholic Church, that it really and actually is the teaching of a man.
Do not keep fooling yourself, your salvation is *not* dependent upon the status of the pope. Your salvation is wholly dependent upon you persevering in the Catholic faith and dying in sanctifying grace, without mortal sin on your soul. You can do this, we can and are all expected to persevere, regardless of the status of the pope, regardless of whether he be a fraud or a living saint.
You can be subject to the pope and you must be subject to him regardless of your opinion of his status - no one gave you the right to declare yourself not subject to him no matter what your opinion of his status is.
The dogma states quite completely;
Furthermore, we declare, we proclaim, we define that it is absolutely necessary for salvation that every human creature be subject to the Roman Pontiff. Madmen changed that dogma into a dangerous doctrine, a doctrine of man, by adding the proviso at the end; ".....be subject to the Roman Pontiff,
unless you don't believe he is the pope".This addition is what the sedevacantists add, while they insist they are doing no such thing. Their reasoning is that the pope is not the pope - as if they actually know this, as if their opinion has been dogmatically defined or a divinely revealed truth - instead of their opinion. Sedevacantists have the highest regard for their own opinion that I know of.
I understand that as a rule, sedevacantists do not differentiate between, "being subject to the pope" and "blindly submitting to the pope", but if you strive to understand the difference, you will discover that as usual, it was with exacting reason that the Holy Ghost used the word "subject" instead of "submit". To use "submit" would not even be Catholic. Hopefully you'll at least think about it.