"Salza and Siscoe quote Billuart (who quotes Martin V’s Ad evitanda scandala) in a futile attempt to refute Bellarmine and the unanimous teaching of the Fathers....."

"Billuart’s error consists in his failure to make a critical distinction between those who lose their jurisdiction as a result of excommunication, and those who lose it ex natura hæresis, as a consequence of defecting from the faith and the Church, and thereby losing office and jurisdiction. Bellarmine points out that the decree only applies to excommunicates."

"The reason why Billuart’s failure to distinguish between those who lose their jurisdiction as a result of excommunication, and those who lose it ex natura hæresis, is of such great consequence, is that the ordinary and habitual jurisdiction of the officeholder is lost upon loss of office due to tacit resignation; but the excommunicates were provided with supplied jurisdiction in virtue of Ad evitanda scandala, and by the subsequent legislation that later replaced its provisions."

"Billuart erroneously deduced that 'heretics retain their jurisdiction', whereas all jurisdiction is lost by heretics, ex natura hæresis; but since heretics incur excommunication latæ sententiæ, jurisdiction was supplied by the decree Ad evitanda scandala. Billuart’s failure to distinguish between retaining jurisdiction and receiving supplied jurisdiction in virtue of the law itself led him into error on the question of loss of jurisdiction of a heretic pope."

"Billuart’s argues that since heretics retain jurisdiction 'for the benefit and tranquility of the faithful', therefore similarly, 'Christ, by a special dispensation, for the common good and tranquility of the Church, will continue to give jurisdiction even to a manifestly heretical pope, until he has been declared a manifest heretic by the Church.' Bellarmine’s words crush Billuart’s thesis: 'I say this avails to nothing. For those Fathers, when they say that heretics lose jurisdiction, do not allege any human laws which maybe did not exist then on this matter; rather, they argued from the nature of heresy.' Hence, there can be no exception by way of a 'special dispensation' from a loss of jurisdiction that results from the very nature of heresy. Heretics do not retain their jurisdiction: Jurisdiction is supplied to latæ sententiæ excommunicated heretics who not only lose all habitual jurisdiction, by their excommunication, but lose it ex natura hæresis. Billuart correctly notes that 'The pope… does not have his jurisdiction from the Church, but from Christ', but the pope would cease to be a member of the Church and lose all jurisdiction from Christ if he fell into manifest heresy; and since the pope cannot incur excommunication for so long as he remains pope, he could not receive supplied jurisdiction from such legislation as Ad evitanda scandala unless he were to fall from the Pontificate by tacit renunciation of office. Only then would he become minor quolibet catholico and accordingly incur excommunication latæ sententiæ, and straightaway receive supplied jurisdiction until his loss of office could be enforced by a declaratory sentence – but he would already have ceased to be pope."

All of the above quotes are taken from:

Kramer, Paul. To deceive the elect: The catholic doctrine on the question of a heretical Pope . Kindle Edition.

No, it means nothing to me when you dishonestly snip off the words which immediately precede and follow (which is tantamount to conceding Alphonsus is against you):

“the Pope cannot be deprived of his authority by the council as if it were above him, but that he is deposed immediately by Jesus Christ, when the condition of this deposition [= the declaration of the council] is carried out as required.^3”

But that Billuart and Bellarmine agree has already been shown in previous pages.

What is important, and obvious to all but the wilfully blind, is that we are dealing here with theological opinion.
...

In this lies the whole drama of the sedevacantist movement. Individuals dogmatising an opinion, setting themselves up against their superiors (eg Sanborn against Lefebvre), dividing Tradition, isolating souls from the sacraments etc... and undoubtedly leading some souls down the path of schism, separating them forever from the Holy See.

Is there an error there or is this quote being misinterpreted (as usual) by Salza and Siscoe?  This part (from Billuart) in the actual quotations marks seems to be a reference to the internal sin of heresy, where he's lost sanctifying grace and can still be head of the Church.  I'd like to see the actual full context of the quote.  Indeed, the head can still function after having lost the "influx of interior faith and charity", but that's not what we're talking about here, but rather loss of membership due to EXTERNAL (public manifest heresy), i.e. a physical severing of the head from the body.  If the head is severed from the body, it can certainly no longer govern / control the body.

What is important, and obvious to all but the wilfully blind, is that we are dealing here with theological opinion.