That chart you posted, parentsfortruth, struck me as bunkum, so I looked it up and it is from a Jesuit in 1951. Why do people, trads and sedes even, act as if they can trust moral theology books from the eve of Vatican II by clergy who went along with Vatican II? It's madness!
It's a mortal sin to question the legitimacy of Pius XI? Hm, I wonder if there is any vested interest in that example... Not if you think he stepped beyond the bounds of what a Pope can do, or if you think he had an invalid election, there isn't. Sedes like John Daly, who posted this list, believe that John XXIII was not Pope but he, just like Pius XI or Pius XII, was universally accepted. What is the difference?
Who was it that first began to say that elections of Popes are "historically certain"? I've heard SJB using this logic, and it has always triggered an alarm bell. Please find me one theologian before the 20th century who has employed this argument, SJB. I wonder if is nothing but an attempt to scare and pressure people into going along with the Vatican II Popes that were at that time being prepared for. The infiltration of the Church is so deep, there is no doubt that many learned ( but twisted ) theologians were harbingers of the "new way."
Because this did not happen overnight -- there are severe problems with the papacies of Benedict XV, Pius XI and Pius XII, three Popes whose legitimacy I accept but who, I believe, may have been working against the Church or enemies of the Church in their political decisions. I would classify them as perhaps ushering in the era of Vatican II, smoothing the road to Vatican II. How else can you explain the smothering of Action Francaise or the Cristero debacle? Pius XI was an absolutely horrible Pope, and if someone questioned his papacy because of these decisions, it would be hard to call them a heretic. I have questioned the papacy of Pius XII but only questioned it -- I don't say he's not Pope. He did not cross a line as clearly as Paul VI did by promoting Vatican II, which I believe is proof a posteriori that Paul VI was not Pope.
Also, how can the existence of a papacy as "historically certain" be reconciled with the fact that the election of a heretic to the papacy is null and void according to cuм Ex Apostolatus? It doesn't work. I don't like the emphasis on immediately pre-VII theologians that you and Johns Lane and Daly traffic in, SJB. There is something wrong about Bellarmine Forums, something that always bothered me, and I think this is it.