If we read Cardinal Billot, we see that there is no doubt that it excludes a universally accepted Pope from being a heretic. "But whatever you finally think about the possibility or impossibility of the aforementioned hypothesis [of a Pope ever becoming a heretic], at least one point must be maintained as completely unshaken and firmly placed beyond all doubt: the adherence alone of the universal Church will always be of itself an infallible sign of the legitimacy of the person of the Pontiff, and,
what is more, even of the existence of all the conditions requisite for legitimacy itself."
https://novusordowatch.org/billot-de-ecclesia-thesis29/ Thus, we know with absolute certainty His Holiness Pope Benedict XVI was truly elected Pope in April 2005, and therefore is not a heretic.
But I don't think Cardinal Billot speaks about the possibility of a "second man being while elected while the reigning Pope sits. That is admittedly a little more complicated. But here's the thing for sede-doubtists, sede-impedists, sede-privationsts, or Bene-vacantists. Can you show us at least 1% of the Bishops, like just some 50 out of 5000, who believe and profess Pope Benedict XVI is still the Pope.
In Ex Quo, Pope Benedict XIV says "it suffices Us to be able to state that a commemoration of the supreme pontiff and prayers offered for him during the sacrifice of the Mass is considered, and really is, an affirmative indication which
recognizes him as the head of the Church, the vicar of Christ, and the successor of blessed Peter"
https://www.papalencyclicals.net/ben14/b14exquo.htm As far as I know, there are hardly even 10 Bishops with jurisdiction who do not do name the Pope as Pope in the canon. Can anyone show evidence to the contrary?
Fr. Hunter is clear that when Bishops all recognize the Pope, he is certainly Pope. Van Noort says this is an example of the OUM making a judgment on a dogmatic fact. If someone wants to make a case that Pope Francis is not the Pope, can he show at least 50 Bishops wit jurisdiction who are still "una cuм" Pope Benedict XVI? The same would be needed to be shown by any competing sede-theorists. It is the acceptance of the Bishops of the Teaching Church that declares it infallibly; Fr. Connell also makes this plain in a 1965 AER article.