Wish someone would interview Sean Johnson about his dramatic conversion to the SSPV.

You could see the gradual progression here, where I think he started with the pre-1955 Holy Week changes ... and then he followed +Vigano closely, collecting much of his material, and then when he saw how Bishop Williamson and some Resistance priests were not hostile to +Vigano's position on Bergoglio being an Anti-Pope, he started to think along those lines.
He actually has advocated "sede-doubtism", which is, ironically, a term I coined entirely tongue-in-cheek because it's a butchery, an amalgam of Latin and English ... though the concept behind it I meant quite seriously.
Early on in our debates there was one time I did nothing more in a post than to quote Archbishop Lefebvre when the latter made a statement sympathetic with SVism, with no comment of my own, and he lashed out in fury, calling me a sodomite LOL.
I've actually developed the notion of "Sede-Doubtism" and refer to it more as "Doubt and Resist" -- D&R, which is actually more what +Lefebvre held. I think that Father Cekada did a disservice by coining the term R&R, as if it were a monolithic thing, rather than there being a continuum regarding degrees of certainty regarding whether Bergoglio, Prevost, and others have been popes. I based this on the notion that most theologians consider the legitimacy of a Pope to be "dogmatic fact", and since +Lefebvre, +Williamson, and +Tissier have all made statements to the effect that it's POSSIBLE that they're not Popes, that means they do not consider their legitimacy to be dogmatic fact, since to doubt a dogma is effectively to deny it (as we're all taught in basic Catechism). Then all the stipulations about "Papa Dubius Papa Nullus" kick in, and the interpretations by various theologians that you're not schismatic if you refuse submission based on some doubt about the putative pope's legitimacy. Earlier on in SSPX history, Fr. Schmidberger used to cite the maxim "melior est conditio possidentis" when asked about SVism, and this translates to the putative Popes having "benefit of the doubt", and I think that's more accurately a reflecton of paleo-SSPX thinking, where, since we're not dogmatically sure, we'll give them the benefit of the doubt and obey them when we can, just in case.