A. No, I don't plan to enter the SSPX though I did accidentally attend an SSPX service in Bath, England about 20 years ago. The only really positive thing I can say about SSPX is that they at least admit that certain Catholic doctrines changed radically with Vatican II.Change can be good -- there was a time when Southern Baptist churches had an official policy of racial segregation and excluded blacks from worship. Thank heavens they had the sense to change and repent of this horrible sin.
B. Are you asserting that Catholic doctrine doesn't change? If you are, then how do you square Unam Sanctum with Vatican II? If one doctrine is true than the other doctrine isn't, no matter how much intellectual dancing one does.
Semper eadem does a death spiral on this topic and papal infallibility is on life support.