God forbid the SSPX ever does.

While you oversimplify the principle (it's not about resistance but about the indefectibility of the Church), this is in fact correct.  Conservative Novus Ordites and Sedevacantists adhere to the same Major, the eminently Catholic principle that legitimate Catholic Magisterium and the Public Worship of the Church cannot become corrupt.  This principle has only been taught by every Pope, Father, Doctor, and theologian throughout the entire history of the Catholic Church ... and has been denied only by Old Catholics, Eastern Orthodox, and Protestants prior to the creation of mainstream "R&R" theology.

By R&R I refer to some modern types who claim falsely to be heirs of Archbishop Lefebvre.  Archbishop Lefebvre himself upheld the principle of the Major that the Papacy is protected by the Holy Spirit from corrupting the Church.  He simply questioned the Minor in terms of how this could have come about.  So those who try to defend their Old Catholic position as being that of Lefebvre are slandering him.

I doubt Siscoe or Salza would screw up very simple Latin, and mistake nulla salus for nullo salus, as I read in one former post.  Salus is a feminine noun and nulla is the only option.  This is basic, junior Latin.

But as for Invincible Ignorance, unless it has escaped me, I have never found that the Church has defined the subject of invincible ignorance.  I know some theologians have theorized as to what it may or may not mean, but we throw this term around as though it is a carte blanche ticket into heaven.  Fr Wathen asked the question many years ago, Why is it that these pygmies in Swahililand, Australia, can know about Coca Cola, but they somehow cannot know about the Catholic Church? 

it sounds like because the libs want to exaggerate the extent of invincible ignorance, Fr. Watgen was denying its existence altogether?

The idea of invincible ignorance came about around the 1500s, due to the explorers discovering America and questioning how so many "innocent natives" could be damned.  St Thomas rejects the notion that an adult could die in such ignorance - before they die, they will either accept or reject the Truth.  Most people's ignorance is culpable, so says St Thomas.

. . . we find it in those who have heard nothing about the faith, it bears the character, not of sin, but of punishment, because such like ignorance of Divine things is a result of the sin of our first parent. If such like unbelievers are damned, it is on account of other sins, which cannot be taken away without faith, but not on account of their sin of unbelief. Hence Our Lord said (John 15:22) "If I had not come, and spoken to them, they would not have sin"; which Augustine expounds (Tract. lxxxix in Joan.) as "referring to the sin whereby they believed not in Christ."

The idea of invincible ignorance came about around the 1500s, due to the explorers discovering America and questioning how so many "innocent natives" could be damned.

Wherefore through negligence, ignorance of what one is bound to know, is a sin; whereas it is not imputed as a sin to man, if he fails to know what he is unable to know. Consequently ignorance of such like things is called "invincible," because it cannot be overcome by study. For this reason such like ignorance, not being voluntary, since it is not in our power to be rid of it, is not a sin: wherefore it is evident that no invincible ignorance is a sin. On the other hand, vincible ignorance is a sin, if it be about matters one is bound to know; but not, if it be about things one is not bound to know.