Well, the problem I have is with some modern R&R distorting and misrepresenting Archbishop Lefebvre's position, which I have no issues with.
He affirms the Catholic truth that the Papacy is guided by the Holy Spirit that that this degree of destruction caused by the Pope is not possible. He says to sedevacantists in one speech, "I agree with you there." He then states that SVism is one possible answer. He's never ruled out SVism as a possibility, but simply felt he didn't have the degree of certainty required to formally come out as an SV.
So Archbishop Lefebvre never denied the MAJOR of the SV position. He simply felt that there could be some unknown factor that could account for what's going on. He went through some possibilities, such as that the V2 papal claimants were being blackmailed, or drugged, or whatever. He said that those were not very likely, but he didn't have the certainty of faith to rule them out and conclude they weren't legitimate popes.
Modern R&R, however, claim that legitimate Popes can corrupt the Church ... and then try to pretend that +Lefebvre supported their position. He did not, except possibly for a span of time between 1980 - 1984 or so. But before then and after then, he remained very open to SVism being correct, but just wanted to defer to the Church's authority to definitely resolve the question.
Well, the problem I see with Pro Vobis stance, is that +ABL did not discuss the "Magisterium," or whether or not is it or is not corruptible. As far as I know, it just wasn't a part of Tradition for him.
It seems to me that this issue is relatively new, and I'm not sure who invented it, maybe it was Fr. Fenton, or Fr. Cekada, or Des Lauriers. Or none of them. The term "Magisterium" is also used by advocates of VII. I know, because I used to argue with them on the old Catholic Answers forum. For them, the Church is built on Scripture, Tradition, and "the Magisterium."
The whole issue or subject of "the Magisterium" seems like a novelty to me, and +ABL wasn't really into novelties, for the most part (the BoD issue aside).