More thoughts:
1) What of the objection to Billot which would say, “Surely Billotcould not have meant a pope becomes a bishop retroactively, despite not being consecrated a bishop!”
2) is an episcopal consecration even truly a new power of Order, or merely an “activation” of the fullness of priestly powers imbued but latent at priestly ordination?
Regarding the first question, I note that the Church has definitely declared definitely invalid sacraments valid by mere juridical authority, and without repeating the sacrament even conditionally.
An example of this would be invalid marriages made valid by “sanatio in radice.”
The sacrament is healed (as Billot would say) in the root by mere juridical authority after the fact.
Regarding the second question, it is a centuries-old debate upon which the Church has not yet pronounced upon, and which you are at liberty to choose.
Now, if the episcopacy is not a new conferral of power, but merely a juridical activation of a latent power conferred at priesthood, then there is no problem with reading Billot’s principle as applying to the healing of a pope’s invalid episcopal consecration, because the universal acceptance which legitimates the election provides the juridical recognition of his episcopacy.
In this case, the teal focus should be on BXVI and Francis’s priestly ordination: If they were validly ordained, then the Church can recognize his episcopacy by the mere fact that it recognized him as pope (healing any defect the moment it did).
Both BXVI and Francis were ordained according to the old rite, so according to this view of episcopal consecration merely activating the episcopal power already latent in the priesthood, there is no concern stemming from the doubt regarding the validity of the episcopal consecration (though admittedly the clock would still be ticking as regards the next pope, who would probably be ordained a priest in the new rite).
Incidentally, I believe Archbishop Lefebvre sided with those who believed that episcopal consecration was merely the activation of power conferred at priesthood, and this realization in turn saves him from the charge of inconsistency as regards doubting the new form, but accepting the new popes (which at any rate did not occur until 14 years after his death).
Perhaps the logic expressed above will/could one day be that which is used by the Church to decide the matter of the precise nature of episcopal consecration?