No, the problem isn't in the specifics of how to resolve the Pope issue. Bellarmine vs. Cajetan, etc.
What's at issue here is the nature of the Church and the relationship between the Magisterium and the faithful.
If one gets that part right, lands in a Catholic place, the details about how to this Crisis resolves theologically can vary.
What I have issues with are the types of issues posted by "Your Friend Colin":
R&Rs think it is acceptable to denounce their church’s
1) ecuмenical councils
2) universal liturgy
3) Sacraments
4) theology
5) canonizations
6) canon law
And basically anything YOU don’t like.
I am no dogmatic sedevacantist, but I am a dogmatic indefectibilist. What some articulations of R&R propose are tantamount to a defection of the Church.
When ALL of the above, namely, the Magisterium, the Mass, the Sacraments, theology, canonizations, and Canon Law can all go corrupt and become unacceptable as a whole to the Catholic conscience, to posit that these things can have emanated from legitimate authority, well, there's nothing left of the Church. To believe that these things can go corrupt is to make oneself no different that Old Catholics, Protestants, and every manner of heretic. There are several condemned propositions saying these exact things.
Father Chazal's position, that these men have lost authority due to manifest heresy, and are "impounded" and "quarantined" and have "no authority," while they remain in possession of the office awaiting removal by the Church, i.e. a Cajetan- and John of St. Thomas- like position, there's no issue with that whatsoever. In fact, his is a compelling Catholic position. Archbishop Lefebvre's position is very similar. +Lefebvre conceded (in a video posted by Father Ringrose as he was going sedevacantist), that these things cannot possibly happen due to the Church's protection by the Holy Spirit. He then speculates about various possible reasons all this could have happend: Paul VI being unfree to act (insane, blackmailed, replaced by a double) ... which he dismisses as unrealistic, and also the sedevacantist hypothesis, which he repeatedly declared to be "possible" ... but all the while deferring to the authority of the Church to "one day" resolve the matter. He very nearly pulled the trigger on sedevacantism, for these reasons, but "prefer
to wait."