As to the Magisterium, that is a handy word to throw around when making Catholic arguments. If I have to bet my soul on whether you and UR are accurate and authentic teachers of the Magisterium, or that John Paul II is, I am going out on a limb here and betting on the pope as the reliable authority.
You may or may not believe that where you are I have been and twenty-five or more years ago decided that the path you are choosing or at least the path chosen by SSPX is an absolute dead end. Ubi Petrus Ibi Ecclesia even when Petrus is John XXIII or Paul VI. I find it absolutely astonishing that anyone calling himself or herself a Catholic finds John Paul II objectionable without a peep about those two but each was actually pope and had the keys of Peter as does JP II. I will chalk up to probable ignorance the general absence of legitimate concern over the papacy of Benedict XV without which none of these conversations would be possible since he cancelled the program of Pope St. Pius X of bringing the modernist heretics to heel. The seeds planted during his papacy burst forth as the weeds of the 1960s which cause you to be tempted to schism.
What might St. Thomas More say as to your notion that it is surely reasonable to be looking askance at the hierarchy and reasonable as well to be entertaining serious doubts. Useful analogies might be made to his discussion with Richard White in which he asked him if Wales were really worth his soul and what he would do when, after all the laws had been leveled in order to more efficiently pursue the devil, he would do when the devil turned on him.
SSPX has stolen the Apostolic Succession for schismatic purposes. That is a second cousin to stealing the Eucharist for nefarious purposes. Stop the charade that your ancestors and mine would have approved the outrageous behavior of the SSPX schismatics. May the next pope offer the SSPX adherents (card-carrying or not) a final opportunity to repent and submit and, if they do not, may he apply the full penalties available to those who think they have a right to defy legitimate authority and still call themselves Catholic while consecrating rebellious bishops, ordaining rebellious priests and encouraging the apostasy of the laity.
You might bear in mind the First Commandment admonition not to have strange gods before him. Someone who commits adultery or theft or murder obviously regards these as more important than God himself since he has forbidden such behavior. Those who insist on the "right" to defiance and disobedience of papal authority as a default position, who nitpick JP II endlessly in their incessant campaign of hatred over his excommunication of their impudent heroes (and that decision proves wiser with each disrespectful response by the schismatics) have their own imagination of "tradition" as the false god whom they put before the one, the only God.
I take it that if you were residing in Red China, you would rush to become a part of the Chinese "Patriotic" Catholic Church which also has apparently obtained the stolen goods of a "valid" but illicit apostolic succession to facilitate the consecration of schismatic bishops, the ordination of schismatic priests, the defiance of the Vatican all because the Chinese "Patriotic" Catholic Church, a collective stooge of the Chinese Communist Party, does after all, say only the Tridentine Mass and is ceaseless in its attacks on and disobedience to the Holy Father, being Traditional Chinese Communists. What else is needed to be the "remnant" of the Church since the end of the "good old days", right?
Serious doubts as to what? Certainly not as to the Faith itself. Christ guaranteed that permanently and not in the form of some silly, self-important pack of schismatics, here, in France or in Red China. If you indulge yourself in doubting the Church, you doubt Christ. If you indulge yourself in the ever-growing fantasy that somehow Christ and the Holy Ghost have slipped up and sent us a heretic as pope because he does not delight YOU or do what YOU want, then you doubt Christ and the Holy Ghost. My preference is for the Tridentine Mass. Nonetheless, I am wearying of 90 minute funereal-pace performances with multiple collects and what not. There are reverent Novus Ordo Masses with first rate preaching available locally which are Masses and not careers.
The exaggerated slow-motion that characterizes many of today's renditions of the Tridentine Mass and causes what ought to be a half-hour Mass to stretch to one and a half-hours so that the devotees can indulge themselves in sloooooow motion and remain a cult of insiders. "If you do not wallow luxuriously in every unnecessary additional ten-minute graft-on", so it goes, "how can you REALLY call yourself Catholic? You can't be as good Catholics as we are! Can't we PLEASE have a 2 1/2 hour Mass to show what liturgical athletes we truly are?"
Your ancestors and mine did not attend such marathon extravaganzas. If those who are in love with the endless renditions are so in love with them, fine. Just see to it that there are Masses available in manageable time frames. I confess to being partially Irish by ancestry and devoted to old Irish notions on such matters such as crisply said Masses.
Every Mass need not be Mozart and Palestrina. There is a place for everything and everything in its place. Every Mass need not be high Mass, or Solemn High. On the Novus Ordo side, there are plenty of abuses but the transformation of the Tridentine into a historical preservation project and one distorted into a megamarathon timewise when it used to be simply a part of our culture as Catholics is also an abuse. Let there be each week at least one low Mass of obligation said crisply and without the full smorgasbord of trappings and maybe you would see more enthusiasm by more people for the return of the ancient rite. As things stand, in many locales where the Tridentine is regularly offered, these abuses cause even the worst Novus Ordo Masses to have better attendance.
I am giving you the benefit of the doubt that SSPX is faithful to the Tridentine rite because I most certainly am not going to patronize the schism to find out despite the constant mischaracterization of Monsignor Perl's letter to somehow claim it a Magna Carta for the schism.