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So, Stubborn, do you think the doctrine of Lumen Gentium is wrong? 

"25. Among the principal duties of bishops the preaching of the Gospel occupies an eminent place.(39*) For bishops are preachers of the faith, who lead new disciples to Christ, and they are authentic teachers, that is, teachers endowed with the authority of Christ, who preach to the people committed to them the faith they must believe and put into practice, and by the light of the Holy Spirit illustrate that faith. They bring forth from the treasury of Revelation new things and old,(164) making it bear fruit and vigilantly warding off any errors that threaten their flock.(165) Bishops, teaching in communion with the Roman Pontiff, are to be respected by all as witnesses to divine and Catholic truth. In matters of faith and morals, the bishops speak in the name of Christ and the faithful are to accept their teaching and adhere to it with a religious assent. This religious submission of mind and will must be shown in a special way to the authentic magisterium of the Roman Pontiff, even when he is not speaking ex cathedra; that is, it must be shown in such a way that his supreme magisterium is acknowledged with reverence, the judgments made by him are sincerely adhered to, according to his manifest mind and will. His mind and will in the matter may be known either from the character of the docuмents, from his frequent repetition of the same doctrine, or from his manner of speaking ...

But when either the Roman Pontiff or the Body of Bishops together with him defines a judgment, they pronounce it in accordance with Revelation itself, which all are obliged to abide by and be in conformity with, that is, the Revelation which as written or orally handed down is transmitted in its entirety through the legitimate succession of bishops and especially in care of the Roman Pontiff himself, and which under the guiding light of the Spirit of truth is religiously preserved and faithfully expounded in the Church.(45*) The Roman Pontiff and the bishops, in view of their office and the importance of the matter, by fitting means diligently strive to inquire properly into that revelation and to give apt expression to its contents;(46*) but a new public revelation they do not accept as pertaining to the divine deposit of faith.(47*)" https://www.vatican.va/archive/hist_councils/ii_vatican_council/docuмents/vat-ii_const_19641121_lumen-gentium_en.html

(1) With regard to Humani Generis, the main point was the teaching of the Ordinary Magisterium (Teaching Authority) of the Roman Pontiffs also requires assent. This is called religious submission: https://en.wikipedia.org/wiki/Obsequium_religiosum

(2) As for the entire Hierarchy supposedly being able to defect, how do you reconcile such an Ecclesia-Vacantism-lite position with the doctrine taught in the Oath against Modernism, which says, among other things, that the Charism of Truth will always remain in the Catholic Hierarchy that has Succession from the Apostles? "I firmly hold, then, and shall hold to my dying breath the belief of the Fathers in the charism of truth, which certainly is, was, and always will be in the succession of the episcopacy from the apostles." https://www.papalencyclicals.net/pius10/p10moath.htm

This is why, imo, good Bishops like e.g. +Athanasius, +Vigano etc should be supported and worked with. The Holy Spirit is working through them, and we know that, according to the Divine Promise, the Charism of Truth remains in the Catholic Hierarchy forever. 

So, Stubborn, do you think the doctrine of Lumen Gentium is wrong?

Absolutely. Although it states some truth, at it's core it is wrong, which means the whole thing is fit for the sewer.
It (25) starts out preaching truthfully what bishops and the principle duties of bishops are. If they are faithful and actually do their duty, only then will they "bring forth from the treasury of Revelation new things and old, making it bear fruit and vigilantly warding off any errors that threaten their flock..."

But as reality demonstrates, there is no divine promise or any guarantee they will do their duty, they can all preach heresy in unison with the pope as we have seen them all do.

(1) With regard to Humani Generis, the main point was the teaching of the Ordinary Magisterium (Teaching Authority) of the Roman Pontiffs also requires assent. This is called religious submission: https://en.wikipedia.org/wiki/Obsequium_religiosum

(2) As for the entire Hierarchy supposedly being able to defect, how do you reconcile such an Ecclesia-Vacantism-lite position with the doctrine taught in the Oath against Modernism, which says, among other things, that the Charism of Truth will always remain in the Catholic Hierarchy that has Succession from the Apostles? "I firmly hold, then, and shall hold to my dying breath the belief of the Fathers in the charism of truth, which certainly is, was, and always will be in the succession of the episcopacy from the apostles." https://www.papalencyclicals.net/pius10/p10moath.htm

#1) Yes, we *must not* limit our submission only to dogmas, but also to, as Pope Pius IX taught in Tuas Libenter: "..this submission must also be extended to all that has been handed down as divinely revealed by the ordinary teaching authority of the entire Church spread over the whole world, and which, for this reason, Catholic theologians, with a universal and constant consent, regard as being of the faith".

#2) The beginning of the quote, the part you did not post states: "Finally, I declare that I am completely opposed to the error of the modernists..." As such, we cannot accept or in any way go along with, follow or obey commands riddled with modernist error no matter where they come from. To obey, go along with or to follow the errors of modernists is to break the oath and contribute to the Modernists' efforts, and we will be judged for this contribution.

The other part you did not quote, is the sentence that follows your quote and is merely an affirmation of the decree of V1: "The purpose of this [oath] is, then, not that dogma may be tailored according to what seems better and more suited to the culture of each age; rather, that the absolute and immutable truth preached by the apostles from the beginning may never be believed to be different, may never be understood in any other way".

When put together, the oath swears to hold the same belief as the Fathers, which belief is the truth of dogmas as decreed by the pope. IOW, here he is talking about dogma, yes and the pope, but we are swearing to uphold truth because it is truth that binds us, not the pope. The truth of dogma binds us, and that is the thing we swear to uphold. It is saying that dogma is defined by the pope, the pope comes from the succession of the episcopacy from the Apostles.

"I firmly hold, then, and shall hold to my dying breath the belief of the Fathers in the charism of truth, which certainly is, was, and always will be in the succession of the episcopacy from the apostles. The purpose of this is, then, not that dogma may be tailored according to what seems better and more suited to the culture of each age; rather, that the absolute and immutable truth preached by the apostles from the beginning may never be believed to be different, may never be understood in any other way".

No, not really. You had sides of the Great Western Schism practically declare holy war on each other, and countless common Catholics even moved city so they could be under who they viewed as the true pope. Among the monarchs, they all picked which pope to follow, and since the popes didn't recognise each other at all, their Churches were clearly separate entities, however similar they may have been.

Yes, really. What history book have you seen it written in that they considered a false claimant and his followers to be another Church?