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Uhm, he (and his predecessors) are clearly teaching Magisterially.  Their teaching is at least "merely authentic" Magisterium.

I am sad to see this thread has become mostly people talking past each other.

So here's an example from Amoris Laetitiae (#300). Sedes tend to say Amoris Laetitia teaches heresies, but beyond arguing whether there is or isn't heresy, shouldn't one consider how and to what extent something is conveyed?

If we consider the immense variety of concrete situations such as those I have mentioned, it is understandable that neither the Synod nor this Exhortation could be expected to provide a new set of general rules, canonical in nature and applicable to all cases. What is possible is simply a renewed encouragement to undertake a responsible personal and pastoral discernment of particular cases, one which would recognize that, since “the degree of responsibility is not equal in all cases”,335 the consequences or effects of a rule need not necessarily always be the same.336

What exactly is magisterially taught to us here? There are no new rules, just an encouragement.

Archbishop himself articulates the problem this poses for indefectibility, and this consideration is what left him open to the sedevacantist hypothesis ...

“…a grave problem confronts the conscience and the faith of all Catholics since the beginning of Paul VI’s pontificate: how can a pope who is truly successor of Peter, to whom the assistance of the Holy Ghost has been promised, preside over the most radical and far-reaching destruction of the Church ever known, in so short a time, beyond what any heresiarch has ever achieved?” (Le Figaro, August 4, 1976)

Archbishop himself articulates the problem this poses for indefectibility, and this consideration is what left him open to the sedevacantist hypothesis ...

Nonsense: 
You invent advocates and sympathizers left and right for sedevacantism because you intuit the weakness of the position.
The fact remains that there were only 2 instances in 25 years of thoroughly recorded and publicized sermons, conferences, articles, books, and interviews in which ABL (rightly or wrongly) seems to acknowledge the theoretical possibility of sedevacantism.
Both those instances followed upon the heels of extremely scandalous Roman acts, but you habitually fail to impute any significance to the fact that ABL never backed those statements later, and certainly never supported sedevacantism habitually (and in fact condemned the position officially).
The proper hermeneutic in understanding the mind of an author is that the exception does not disprove the rule (ie., it cannot be used to contradict his normal, general, and overwhelmingly common position regarding sedevacantism).
It is therefore dishonest to attempt to magnify these two particular instances as though they evince a general tolerance (or even sympathy or promotion) of sedevacantism.