A sedevacantist like Nado, for example, needs to show that at least one bishop, but probably much more (since we are talking only of moral unanimity of acceptance) who openly contested Pope Paul VI before the end of 1965, otherwise the sedevacantist explanation of the Second Vatican Council is incorrect.
No, there's no one to be found before 1965 who doubted Paul VI. But there's some evidence that immediately after the 1958 election Catholics were questioning the results. But in 1965 there was no data available to make a judgment on Paul VI Montini.
By the early- to mid- 1970s you had several bishops, most notably Archbishop Lefebvre and Bishop de Castro Mayer who clearly did not have the certainty of faith regarding the legitimacy of these Popes. If you wish, I can back this up. Archbishop Lefebvre tolerated the opinion of sedevacantism, and even at several point mused about whether or not some day they would have to come to the sedevacantist conclusion. Archbishop Lefebvre at one point famously said, in what came across as a tongue-twister in French, that he did not say that Paul VI was the pope and that he did not say that Paul VI was not the pope. Sede-doubtist. Bishop de Castro Mayer also reportedly was uncertain about their legitimacy and tolerated the opinion. Bishop Tisser has recently speculated that it may be acceptable to hold as a private opinion. Bishop Williamson also said that it's not impossible that the See is vacant.
Now, the problem here is that as soon as you say that it's POSSIBLE that the See is vacant, you're saying that you do not have the certainty of faith regarding the legitimacy of that pope. And that's where "Papa dubius, papa nullus." applies. Only popes that are known to be legitimate with the certainty of faith can really function as popes.
But here's the real kicker.
What basically defines Traditional Catholicism is a RECESSION from these Vatican II popes as a rule of faith. And the treatises which deal with the peaceful acceptance criterion always say that it's because it would not be possible for all the world's bishops to adhere to a false rule of faith. In point of fact, the entire Traditional Catholic world has REJECTED the Vatican II popes as rules of faith. No Traditional Catholic is sitting here awaiting the up-coming encyclical of Francis with an obseqium religiosum
a grave presumption of truth regarding its contents. In fact, most of us fully expect 95% of it to be incompatible with Catholic principles. EVERY Magisterial Act of the V2 Popes is viewed with suspicion and a presumption of being polluted with modernism by the entire Traditional Catholic world.
So the R&R can flap their gums all you want about how the V2 Popes MUST be legitimate popes, but it's just hollow lip-service. In no way do R&R have the proper Catholic disposition towards their authentic Magisterium.
And even the non-infallible Magisterium (and discipline) of the Church must be regarded by Catholics as infallibly safe, i.e. that they cannot lead to the ruin of souls.
Consequently, nothing from the Magisterium, once the legitimacy of the a pope has been established a priori
with the certainty of faith can possibly justify refusal of juridical / canonical submission to the Popes.
There's no principle in Catholic theology that allows Catholics to set up churches, set up institutions, and confer the Sacraments which require jurisdiction and the power of the keys for validity (e.g. Confession and Matrimony).
If you believe with the certainty of faith that the V2 Popes are legitimate popes, the only Catholic attitude is to come into juridical / canonical submission to them. Period. There's nothing else to do.
As I said, R&R flaps their gums about the legitimacy of the V2 popes, will even put up a picture of the current claimant in a vestibule, but in no way, shape, or form are they in submission to these popes. You DO NOT accept these men as a rule of faith.
And, along the lines of what I wrote earlier, in citing Monsignor Fenton, it's INCOMPATIBLE WITH THE INDEFECTIBILITY of the Church at large to claim that it could ever happen that the Church could have an Ecumenical Council addressed to the Universal Church and promulgate a form of worship that must be rejected in conscience. Then the CHURCH WOULD HAVE DEFECTED. I remind you that the REASON for the "peaceful acceptance" principle comes from the indefectibility of the Church because then the Church could be led into error. But then HOW has the VERY SAME episcopal body been led into error by submission to the Magisterium when that too is incompatible with the Church's indefectibility.
In the conclave that elected Pope St. Pius X, there was external interference as well.
In the St. Pius X conclave, the interference was canonically sanctioned. St. Pius X immediately upon election abrogated this veto power, but it was in fact in force during that conclave. So that's apples and oranges.