The sedevacantists must observe that the Catechism of Pope Pius X is Feeneyite. See numbers 24Q and 27Q.1
However the confusion for the sedevacantists comes with 29Q.2
If 29Q is assumed to refer to visible cases in the present times; personally known people saved in invincible ignorance and without the baptism of water in the Catholic Church, then 29Q contradicts 24Q and 27Q.This is the Cushingite interpretation of the Catechism of Pope Piux X.There are exceptions for Cushingites.
For me 29Q refers to an invisible case.So it is not an exception to 24Q and 27Q.So the Catechism of Pope Pius X is Feeneyite and not Cushingite for me.
It is with this rational reasoning that I interpret Vatican Council II as not being a rupture with the Catechism of Pope Pius X, the dogma extra ecclesiam nulla salus as it was known to the 16th century missionaries, the Syllabus of Errors and the rest of Tradition.
For the sedevacantists Vatican Council II is a rupture with Tradition and so they went into sedevacantism during the pontificate of Pope John Paul II or earlier.
So now they know the error. They can correct it.Vatican Council II supports the old ecclesiology.
24 Q. To be saved, is it enough to be any sort of member of the Catholic Church?
A. No, to be saved it is not enough to be any sort of member of the Catholic Church; it is necessary to be a living member.
27 Q. Can one be saved outside the Catholic, Apostolic and Roman Church?
A. No, no one can be saved outside the Catholic, Apostolic Roman Church, just as no one could be saved from the flood outside the Ark of Noah, which was a figure of the Church
29 Q. But if a man through no fault of his own is outside the Church, can he be saved?
A. If he is outside the Church through no fault of his, that is, if he is in good faith, and if he has received Baptism, or at least has the implicit desire of Baptism; and if, moreover, he sincerely seeks the truth and does God's will as best he can such a man is indeed separated from the body of the Church, but is united to the soul of the Church and consequently is on the way of salvation
Letter of the Holy Office 1949 to the Archbishop of Boston contradicts the Catechism of Pope Pius X
Catechism of Pope Pius X : Difference between John Vennari and me
I affirm Vatican Council II with the old ecclesiology of the Catechism of St. Pius X