Ladislaus, I think we started a similar debate but never finished it.  Here's my question:  Wouldn't the "doubt" about the legitimacy of the pope have to be a legitimate doubt in the first place?  I.e.  If I doubt the pope is pope because someone told me that a papal election has to happen on Wednesday and +Benedict was elected on Tuesday.  In this case, sure, I have a doubt, but the doubt is not based on correct info.
.
In the same way, most Trads who have doubts about the post-V2 popes did NOT have doubts when the popes were elected, but Trads only doubted AFTER the election, and Trads based such doubts on the popes' heretical acts/words.  I say that this kind of "doubt" is misplaced because the theologians at the time of +Bellarmine always held open the possibility that a pope's private faith could fail and he could be declared a heretic.  Hence, the idea that the post-V2 popes aren't popes because of a lack of orthodoxy is NOT A VALID REASON TO DOUBT THEIR LEGITIMACY.  Only the Church can rule on their legitimacy.
.
So, being that most Trads follow this faulty reasoning, then i'd say their "doubts" do not affect Universal Acceptance because their doubts are wrong.
.
If one wants to argue that some/all of the post-V2 popes were Freemasons and not eligible for election, then ok.  But I'd counter that St Pius X and Pius XII changed the election laws, so that doubt is invalid too.
.
The only rational doubt that has only facts to support it is for Paul VI, which one could doubt based on the Siri Thesis.  But, any pope JPII and after is not affected by this.

Mate I'm actually trying to defend Universal Peaceful Acceptance vs Stubborn here. I was just trying to point out that he was misusing/misintepreting his quote, and in the process he pretty much declared that it's impossible an election could ever be invalid. Well the first John XXIII proves that it can, as does canon law but he ignored that point when I tried it. I didn't bring up the possibility of invalid elections as proof of sedevacantism here, but rather as proof he was misinterpreting the quote.

I get your confusion since I argued a bit about the mechanics of it earlier, but I conceded the point there and now I'm trying to correct Stubborn's illogical arguments against it. I still have a lot more to read up on it myself, but it doesn't take knowing and agreeing with something for certain to see when arguments against it are fallacious.