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Accusing sedevacantists of mixing infallibility with indefectibility is just dodging the issue with the latter by attacking the strawman of the former. Fact of the matter is, if Vatican 2 is a legitimate Church council and Francis is a legitimate pope, then the Church has been taken over by non-Catholics and has promulgated teachings and rites which are harmful to faith - by definition, a defection.

No, BOTH sides are conflating infallibility and indefectibility.  Your second sentence here is in fact the argument from indefectibility.  There's no need to exaggerate the scope if "infallibility in the strict sense" (as Msgr. Fenton called it).

That's not catholic at all.

He's talking about with regard to his official papal acts, the Magisterium and Universal Discipline of the Church ... not private commands.

So what of this notion that Vatican II was a pastoral Council that did not intend to teach anything new?

Its intention was to re-present existing Catholic teaching to the world in a way that would make it more palatable.  It's a re-interpretation.  Sure, the Church teaches that there's no salvation outside the Church, but, hey, you all got it wrong, what the Church REALLY meant was ...  Oh, of course we have a Mass, but it is in fact the "Lord's supper" and "assembly of the faithful."  People used to think that when we taught that the Catholic Church was the ONE TRUE Religion, it meant that other religions were false, but, no, what we REALLY meant was that we have the FULLNESS of truth, not that you guys are wrong or bad.  You all have truth too.  We're both true.  You've heard it said that ..., but what we REALLY MEANT was ...

Basically, the very notion of this "Pastoral" Council was MODERNISM IN A NUTSHELL.  It's the notion that the meanings of Catholic doctrine can change with time and be MODERNIZED.

SO THE VERY NOTION of a PASTORAL Council is in fact textbook Modernism.  Consequently, it is heretical in its very intent and formal end, in its entire orientation.  It is not just one or two bad propositions.  You could strike 10% of it from the existence, and it still would remain a blight and a pollution on the Magisterium.  Who could ever take the Catholic Church seriously ever again?  It reduces papal Magisterium to a mere opining about various subjects related to the faith which any Catholic is free to take or leave if he decides that it's contrary to prior Church teaching.

Sure, let's keep Vatican II as a legitimate Council.  Pius XIII comes along and overturns some teachings of Vatican II.  What's to stop a Catholic from saying, Pius XIII got it wrong because he went against Tradition (as in the Vatican II Tradition)?  In that case, the only way a Pope would have to put an end to dissension and nonsense would be to issue solemn pronouncements, because everything short of that is fair game for dissent.  Yo, Pius X, you condemned Modernism?  Well, I think you're wrong.  Phouey on you!  Until you issue a dogma, I'm going to keep on truckin'.