CathInfo.com

I'll play.  Of course, I'll start by correcting your term sede-ism, because sedeplenism is also a sede-ism.

If I became convinced with the certainty of faith that Bergoglio is the legitimate Catholic pope, then I would return to full communion (to use their term) with the Catholic hierarchy while working out my understanding the New Mass and Vatican II through the appropriate lense.  I would hold the New Mass and other Sacramental Rites to be unequivocally valid.  I would still find a Traditional form of Mass to attend, e.g. FSSP, Motu, or Eastern Rite variant.  But I would return to full communion with the Church at that point.  I would continue to apply the hermeneutic of continuity to Vatican II, because I find it impossible that an Ecuмenical Council of the Catholic Church could contain substantial error.  In other words, I would reject the positions of both +Schneider and +Vigano who claim that there's error in a legitimate Council of the Catholic Church.  I would avoid even the SSPX, since it would be wrong to give the impression that it's OK to continue in a state of separation from the legitimate hierarchy.

Of course, I have pointed out repeatedly that neither +Lefebvre nor +Tissier nor +Williamson have held (and do hold) that it's certain with the certainty of faith that the V2 papal claimants are popes, and that is the justification for their separation from the Conciliar Church, and it is also my own.  So while they are not sedevacantists, they are not sedeplenists either.  They're sede-doubtists.

You, on the other hand, claim that it's certain with the certainty of faith that these men are legitimate Popes and somehow feel it's OK to remain supportive of an organization that is not in full communion with the hierarchy (to use the modern term).

Sedevacantism is at best an opinion only. It is not a dogma; ... Sedevacantists have forgotten what it means to say "to the best of my knowledge and judgment, such and such seems to have happened. Nevertheless, I am not infallible; and, if the Church judges otherwise, I retract my opinion and submit to the judgment of the Church

OK, but many if not most sedevacantists would agree with this.  Why do you lump all sedevacantists in with the dogmatic sedevacantists?  You're setting up a false strawman and categorically lumping all sedevacantists in with the dogmatic fringe.  Most of us realize that it's just a private opinion without any authority behind it.  In fact, Bishop Sanborn wrote an entire article condemning what he called the "opinionist" sedevacantists.

I just wrote to you what my response would be if I believed with the certainty of faith that Bergoglio is the pope.  Many/most sedevacantists would say the same thing.  You see 3 people who already responded in your poll that they would become either Indult Traditional or Conservative NO Catholics.  I voted for the latter section, but there's a blur there.  I opted for the latter, because although I would still personally prefer the Tridentine Mass, I would no longer hold that it's in any way objectionable per se to attend the NOM.

I would NOT, however, take your position, where you think it's OK to be out of full communion with the legitimate Catholic hierarchy.  Your position makes zero sense.

Only if you think there is no such thing as the “authentic magisterium “ (ie., teachings “promulgated” by those invested with office, but which are non-magisterial due to lacking universality of time; such teachings are ipso facto those of private doctors which have gained widespread assent and a counterfeit officiality by using the organs of the Church for diffusion).

Fr. LeFloch (Lefebvre’s seminary rector in Rome) predicted the error of sedevacantism back in 1926:

“The heresy which is now being born will become the most dangerous of all; the exaggeration of the respect due to the pope and the illegitimate extension of his infallibility.”

http://archives.sspx.org/miscellaneous/infallible_magisterium.htm

I assume you mean "MERELY authentic" Magisterium.  Infallible Magisterium is also authentic.  That's the wrong question, Sean.  Of course there's merely authentic, i.e. non-infallible Magisterium.  And R&R vs. sedes have been arguing the wrong issue all these years.  It isn't about infallibility in the narrow sense but about the overall indefectibility of the Catholic Church.  If we were talking about a couple minor points here or there in Vatican II that required some amendment, I'd have little issue.  But if that's what we were talking about, there would be no Traditional movement in the first place.  If the Catholic Magisterium and Universal Discipline could go so badly off the rails as to justify and even require a Traditional movement, then the Magisterium and Universal Discipline of the Church would have failed.  If the Magisterium, authentic or otherwise, were capable of leading souls to hell, then it's worthless and we might as well be Protestants.  This is about indefectibility and not infallibility.  By asserting that the official Church teaching (infallible or not) could fail on so grand a scale as to justify the Traditionalist response, you're undermining the Church's indefectibility.

The Magisterium and Universal Discipline of the Church must be considered infallibly safe:

Monsignor Fenton:

In this field, God has given the Holy Father a kind of infallibility distinct from the charism of doctrinal infallibility in the strict sense. He has so constructed and ordered the Church that those who follow the directives given to the entire kingdom of God on earth will never be brought into the position of ruining themselves spiritually through this obedience. Our Lord dwells within His Church in such a way that those who obey disciplinary and doctrinal directives of this society can never find themselves displeasing God through their adherence to the teachings and the commands given to the universal Church militant. Hence there can be no valid reason to discountenance even the non-infallible teaching authority of Christ’s vicar on earth.
...
It is, of course, possible that the Church might come to modify its stand on some detail of teaching presented as non-infallible matter in a papal encyclical. The nature of the auctoritas providentiae doctrinalis within the Church is such, however, that this fallibility extends to questions of relatively minute detail or of particular application. The body of doctrine on the rights and duties of labor, on the Church and State, or on any other subject treated extensively in a series of papal letters directed to and normative for the entire Church militant could not be radically or completely erroneous. The infallible security Christ wills that His disciples should enjoy within His Church is utterly incompatible with such a possibility.

In order to rebut the R&R position that the Magisterium and Universal Discipline of the Church can defect, many sedevacantists have exaggerated the scope of infallibility beyond what any Catholic theologians prior to Vatican II ever held.  It's because they're arguing the wrong point and mistaking the broader indefectibility question with the notion of infallibility "in the strict sense" (as Mgr. Fenton referred to it).