You'll need to define what you mean by the Conciliar Church.  If you mean the visible society comprised of the local Church of Rome and the diocese and eparchies throughout the world in union with it, that is indeed the indefectible Catholic Church founded by Christ, and to deny it is heresy. So hopefully that's not what you are referring to as the Conciliar Church. 

If the visible society described above was the indefectible Church with four marks in 1958, it is the indefectible Church with four marks today.  If it's not the indefectible Church with four marks today, it never was.  Christ's promise that "the gates of hell shall not prevail" applies to the visible, hierarchical society, as such.  It doesn't mean there will always been a remnant of "true believers," which is what the Protestant heretics claim. 

By your principles, those faithful Catholics who during the Arian crisis rejected the Arian usurpers, and the saintly bishops who went around consecrating parallel Catholic bishops to replace them, they were all outside the Church, while the Arian usurpers were in rightful possession of their sees.  In fact, by your principles, had the Arians succeeded in getting one of their own onto the See of Peter, the Arians would all be inside the Church and the true orthodox Catholics outside.  I hope you can start to see the absurdity of your purely legalistic view of the Church.  And, yes, St. Athanasius did hold that even if the Church were reduced to a handful of faithful, there would be the Catholic Church.

Our Lady of Lasalette said “the church will be in eclipse”.  

Mélanie, I will say something to you which you will not say to anybody:
 
"The time of the God's wrath has arrived!
 
"If, when you say to the people what I have said to you so far, and what I will still ask you to say, if, after that, they do not convert, (if they do not do penance, and they do not cease working on Sunday, and if they continue to blaspheme the Holy Name of God), in a word, if the face of the earth does not change, God will be avenged against the people ungrateful and slave of the demon.
 
"My Son will make his power manifest! Paris, this city soiled by all kinds of crimes, will perish infallibly. Marseilles will be destroyed in a little time. When these things arrive, the disorder will be complete on the earth, the world will be given up to its impious passions.
 
"The pope will be persecuted from all sides, they will shoot at him, they will want to put him to death, but no one will be able to do it, for the Vicar of God will triumph again this time.
 
"The priests and the Sisters, and the true servants of my Son will be persecuted, and several will die for the faith of Jesus-Christ.
 
"A famine will reign at the same time.
 
"After all these will have arrived, many will recognize the hand of God on them, they will convert, and do penance for their sins.
 
"A great king will go up on the throne, and will reign a few years. Religion will re-flourish and spread all over the world, and there will be a great abundance, the world, glad not to be lacking nothing, will fall again in its disorders, will give up God, and will be prone to its criminal passions.
 
"[Among] God's ministers, and the Spouses of Jesus-Christ, there will be some who will go astray, and that will be the most terrible.
 
"Lastly, hell will reign on earth. It will be then that the Antichrist will be born of a Sister, but woe to her! Many will believe in him, because he will claim to have come from heaven, woe to those who will believe in him!
 
"That time is not far away, twice 50 years will not go by.
 
"My child, you will not say what I have just said to you. (You will not say it to anybody, you will not say if you must say it one day, you will not say what that it concerns), finally you will say nothing anymore until I tell you to say it!

That's not in the original approved message.  It comes from later version written in 1879, which was placed on the Index.  Here is the original, approved message of LaSallette:


Nothing about Rome losing the faith and becoming the seat of Antichrist, or about the Church being eclipsed, in the original approved message.
 

Everything that was de fide before Vatican II is de fide today, and nothing has been proposed as de fide since Vatican II that was not already de fide before the Council.  What that means is that the faith taught by the Church has not changed. 

Among the prerogatives conferred on His Church by Christ is the gift of indefectibility. By this term is signified, not merely that the Church will persist to the end of time, but further, that it will preserve unimpaired its essential characteristics. The Church can never undergo any constitutional change which will make it, as a social organism, something different from what it was originally. It can never become corrupt in faith or in morals ...

It is manifest that, could the storms which the Church encounters so shake it as to alter its essential characteristics and make it other than Christ intended it to be, the gates of hell, i.e. the powers of evil, would have prevailed. It is clear, too, that could the Church suffer substantial change, it would no longer be an instrument capable of accomplishing the work for which God called it in to being. He established it that it might be to all men the school of holiness. This it would cease to be if ever it could set up a false and corrupt moral standard.

“The Church of Christ, by the revelation and institution of Christ himself, is essentially visible; and this visible Church is the Church to which his promises pertain; promises, namely, that she would be perennial and indefectible, and that in her and by her men would find sanctity and salvation.” (Billot, Tractatus de Ecclesia Christi, vol. 1, 3^rd ed. 1927, )