MAJOR:  Legitimate Popes, protected as they are by the Holy Spirit, cannot destroy the Church in this manner (Archbishop Lefebvre himself affirmed this).

This is not what Archbishop Lefebvre affirmed.

A pope can contribute to the destruction of the Church, only as a private person.  Observe how none of these popes invoked infallibility for their errors.

Pffft.  I doubt the man was ever a sedevacantist.  You don't go from SVism to being an R&R attempting to cozy up to Rome.  If anything, you go straight back to the Conciliar Church.

Yes, the MAJOR premise of SVs and conservative Novus Ordites is the same, and they differ on the MINOR.

SVs --

MAJOR:  Legitimate Popes, protected as they are by the Holy Spirit, cannot destroy the Church in this manner (Archbishop Lefebvre himself affirmed this).
MINOR:  Conciliar "Popes" have destroyed the Church.
CONCLUSION:  Conciliar "Popes" are not legitimate Popes.

Conciliar Novus Ordites do in fact accept the MAJOR (as did Archbishop Lefebvre).  They reject the MINOR by explaining away or minimizing the Post-V2 destruction.

R&R and "liberal" Novus Ordites reject the MAJOR.  Where they differ is that R&R accept the MINOR, while Novus Ordites don't think V2 represents any kind of "destruction" but is great stuff.

This isn't based on "legalism", as Sean claimed, but on some very profound theological principles.  There's nothing "legalistic" about the MAJOR.  Archbishop Lefebvre himself affirmed it.

What Archbishop Lefebvre did, however, was to distinguish the MAJOR, and he was correct to do so.

MAJOR:  Legitimate Popes, protected as they are by the Holy Spirit, who are of sound mind and acting freely, cannot destroy the Church in this manner.
MINOR:  Conciliar "Popes" have destroyed the Church.
CONCLUSION:  Maybe SVism, maybe something else, such as blackmail or some other unknown factor (he bandied about the imposter pope or drugged pope theories).

Archbishop Lefebvre was correct in adding this distinguishing to the MAJOR, and given that we don't have the certainty of faith regarding whether Montini et al. were somehow coerced, blackmailed, or even (with Montini) replaced ... we don't have the certainty of faith regarding the conclusion.  Conclusions are only as strong as their premises, and we can only know this CONCLUSION with the same certainty as we can rule out some other nefarious situation short of the Conciliar Popes being strictly illegitimate.

As I said, I have no problems with this, because the Archbishop affirmed the MAJOR above:

Legitimate Popes, protected as they are by the Holy Spirit, who are of sound mind and acting freely, cannot destroy the Church in this manner (as we have seen with V2).

Here we go again.

Then what happened to Fr Schmidberger?  He was sede, but still was voted in as Society Superior General as a sede?  Or did he switch from sede to R&R?  And if the latter, I don't understand why his sedevacantism in the early years were even brought up/is relevant.

Happens all the time.  The overly legalistic mind of the sedevacantist compels one or the other, but unfortunately precludes a more nuanced understanding (i.e., R&R).

That's why entire sedevacantist communities reconcile with Rome, like the formerly sede Dominican community, the Fraternity of St. Vincent Ferrer:  https://en.wikipedia.org//wiki/Fraternity_of_Saint_Vincent_Ferrer

Same thing with the Sisters of Mary, Mother of the Church: https://www.ncregister.com/news/sedevacantist-sisters-reunite-with-church

Or a big chunk of the CMRI sisters: https://www.catholicworldreport.com/2012/10/19/the-return-to-rome-five-years-later/

These examples seem to support Lad's point.  They didn't go via R&R/SSPX (like Fr Schmidberger).  They went directly back to the Conciliar Church.

I am not familiar with Fr Schmidberger, the "then-Superior General" of the SSPX? If he was a sedevacantist, then why would he be eager for an agreement with modernist Rome? Typically, sedevacantists want anything but. It also seems odd that the Superior General of the SSPX would be a sedevacantist. Do you (or PV) have support for his being a sedevacantist?