Why is it that those clergy that subscribe to the thesis hold that sspx mass attendance is objective mortality sinful?  What is the difference between una cuм mortal sin and sspv positive doubt regarding certain consecrations?
Both parties are essentially pope and become the final judgment regarding access to the sacraments.
These clerics pose no threat to actual heretics  but rule with an iron fist with the most affable faithful.

By the way, as usual, I missed the part where it's explained how exactly sedeprivationism solves the epistemological problem.

It doesn't.  That's why I have a different position in terms of the role of individual judgment that is not primarily focused on the legitimacy of a given pope, but, rather, on identifying the True Church of Christ.

John of St. Thomas articulated the quoad se vs. quoad nos distinction, arguing that the quoad se is effectively subordinate to the quoad nos, which I have come to realize is a seriously defective position.  It's almost like a theological phenomenology.

As Vatican I taught about the process of supernatural faith, human beings use natural reason to identify the True Church founded by Christ from the rational motives of credibility, and then this Church determines who holds authority in it.

Traditional Catholics have come to the conclusion that the Conciliar Church lacks the Marks of the True Church of Christ and lacks those motives of credibility, that aspect of the process of faith that is rooted in natural reason.

Imagine for a second that Vatican II had never happened, and that there had never been a New Mass.  Within this Church, then, we had a Jorge Bergoglio come to office.  He doesn't teach error or heresy, but he's spouting heresies in his interviews with Scalfari or on his papal plane trips.  In that kind of scenario, it's not our concern, nor is it our problem, and not our duty to address.  We let the Cardinals handle it.

Another scenario that sheds light upon the matter is this.  Let's look at one Cardinal Richard Cushing.  That guy was as manifest a heretic as they get.  "No salvation outside the Church?  Nonsense." ... and that's just the tip of the iceberg.  Did he remain the Cardinal/Archbishop of Boston?  According to straight SVism, he did not.  Yet most of the dogmatic SVs hold that he rightly disciplined Father Feeney.  But by SV criteria, the man was not the Archbishop of Boston.  Or was he?  He was never removed from office by the Pope.  Priests continued to receive jurisdiction from him for the Sacraments.  And so forth.  SVs need to answer what the status of a pre-Vatican II manifest heretic bishop who was not deposed by Rome (there were many such) ... and that status is analogous to the status of the post-V2 papal claimants.

At the time, there was a lot of FUD regarding what took place at the various "elections," and a lot of material error regarding the situation, but an analysis of the facts in hindsight (which is famously said to be 20/20) reveals the truth.

We have other scenarios that are problematic, 2 or 3 different situations where a living Pope was taken into exile, and a successor elected to replace him and "universally accepted" as such, while the pope was still a live, and in one case where the living pope had openly protested his replacement.  Clearly the original pope was still the pope because the Church cannot depose a pope.  This militates against the notion of "universal acceptance" establishing dogmatic fact.

I'm inclined to agree with the theologians who hold that the legitimacy of any given pope can only be knowable with a moral certainty, and that where the dogmatic aspect comes into play is the Church's acceptance of him as the rule of faith, as even Billot, one of the most famous proponents of UA, admits.

Can you go more into details of this?