Ladislaus, thanks for your questions and thoughts, I do not know if I can address all of them here. I agree indefectibility is a serious consideration that needs to be taken into account by all sides, but where sedevacantists err in this matter is in trying to use indefectibility in reverse, what you have justly syled modus tollens sedevacantism. Now, indefectibility was never meant to be applied in this manner and even the attempt to do so is heterodox.
After we have seen that we must recognize the Pope, it is only a matter of consistently gauging what indefectibility does and does not entail. In other words, it is no longer between SVism and "R&R", but rather between the SSPX position and a more FSSP-like position. The SSPX is accused of denying the indefectibility of the Church, but once the appropriate distinctions are made, it will be seen that it does not. Everything since the Council only requires the obsequium religiosum, and the Society renders this submission to the Roman authorities. The SSPX is not outside the communion of the Church.
We insist only on two rights, that have often been acknowledged privately, and sometimes publicly by Rome, (1) The right of all priests to offer exclusively the traditional Mass, as also the right of all the Catholic faithful (including those affiliated with indultarian societies) to assist exclusively at the same (2) The right to critique the Council on points where it is shrouded in ambiguity or imprecision, which lead to error in practice, with Tradition and prior Magisterial teaching being the judge and criterion. If you are having doubts or scruples about whether the Society and traditional Catholics who support it are in "full communion" with Rome, please read this, which Bishop Fellay mentioned some years ago, "Then there are painful cases that concern sins so severe they are penalized by excommunication reserved only to the Pope. SSPX priests who confront these cases in the confessional absolve the penitent from the sin, and from the excommunication. According to Church policy, the priest must then send the case to Rome to be examined, and the excommunication formally lifted. Bishop Fellay says, “Every time – absolutely every time – we have received an answer from Rome that the priest who took care of this confession did well, that it was perfectly in order, and it was both licit and valid.” Rome would then comment on the penance, whether it was sufficient or not enough."
For the sake of brevity in the matter of universal acceptance, we can limit ourselves to examining whether the Pope was universally accepted by the ecclesia docens or episcopate (1) in 1965 and (2) in 2015 and if so, what are the implications of this vis-a-vis sedevacantism. The ecclesia docens is indefectible.
The first question all Catholics must ask themselves is "Where is the ecclesia docens today?" It is taught by Vatican I, and unanimously by all theologians, including those present there, that a visible hierarchy of bishops sent or appointed to episcopal office by a Pope must always exist. Most of those who hold to 57 year sedevacantism even hold there is no hierarchy anymore, since almost all the bishops appointed by Pope Pius XII have died or left office, but that theory (which is more "ecclesia vacantism" than sede vacantism) is heretical, and good Catholics should not hold it. The second question, "Does the ecclesia docens recognize the Pope?" If yes, then these two considerations taken together make 57 year sedevacantism untenable. Can I ask your answer to these two questions, my friend? If you agree the bishops with ordinary jurisdiction who constitute the visible hierarchy today, with moral unanimity, recognize the Pope, then what Fr. Hunter says applies, "the Church is infallible when she declares what person holds the office of Pope ...This argument is in substance the same as applies to other cases of dogmatic facts ... it is enough to say that if the Bishops agree in recognizing a certain man as Pope, they are certainly right, for otherwise the body of the Bishops would be separated from their head, and the Divine constitution of the Church would be ruined."
Please don't stop posting, Ladislaus. I think you know that your informative contributions are much valued here, by me and other posters.