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So, I know that Stubborn here constantly pushed this position and for the longest time I told him that no theologian ever held that position.  I was wrong.  Msgr. Fenton did a historical survey of these and found one theologian in the 19th century, but the position pretty much died with him ... AND Pius XII pretty much put the nail in its coffin with Mystici Corporis, where he affirms the position that heresy and schism sever from the Church.

Now, one might still argue manifest vs. occult, and ipso facto or based on some declaration (i.e. Bellarmine vs. Cajetan), etc.  But there's absolutely no one out there who holds "Once a Catholic, always a Catholic." -- despite it having been revived by Father Wathen and his zealous followers (like Stubborn).  They'd be much better off arguing what type of heresy removes one from membership in the Church and what it takes to become that kind of heretic.

Yeah, I would love to interview Fr Wathen (RIP) and get his complete thoughts on the matter.  Because his views in books seemed to be...unfinished.  He certainly called all the V2 popes heretics, yet then argued that they were baptized and still members of the Church.

He was definitely arguing against the rabid dogmatic Sedes of his day; that was his purpose in even bringing up the topic.  He thought the idea of Aunt Sally waking up and declaring someone 'not in office anymore' as ludicrous and anti-canon law.

Had he spent more time explaining his view, I suspect he would've come to the conclusion of manifest vs occult heresy...which is similar to Fr Chazalism/sede-impoundism...since anyone who is guilty of occult heresy (i.e. private/personal heresy) is in mortal sin and loses their spiritual authority in a number of ways, as Canon Law specifies.  And technically, until the Church declares someone as a heretic, then they are occult (and not manifest).

Ultimately, all roads lead to a 'dead end' in this crisis, even if some roads go further and explain more than others.  Until Christ restores his Church, and fixes the broken roads.

Right ... I agree that we were missing some nuances in his thinking (and perhaps Stubborn is also).  That was certainly the case with Archbishop Lefebvre where the nuance was lost and his followers morphed.

So Archbishop Lefebvre's position (apart from that unfortunately stint in the early 1980s) was never dogmatic-anti-SV, and if you read what he said, for him it was just because he wasn't totally sure.  In fact, the SSPX rhetoric at the time was epitomized by Fr. Schmidberger's citation of the canonical principle melior est conditio possidentis ... which essentially means that we're giving them the benefit of the doubt, i.e. until the Church intervenes we're going to act like they might be or likely are legitimate popes.  After the Archbishop passed away, though, this gradually morphed into a quasi-dogmatic R&R that +Lefebvre never held (again, except in the early 1980s perhaps). 

So, as you said, I imagine we're missing some nuances in Father Wathen's thinking, and could very well have been moving along the lines of where Fr. Chazal ended up, just hadn't been able to articulate it.  It does sometimes happen where there's something in your mind, but where at least for a while you can't quite put it into words, where you can't find the right distinction to explain it.

I just don't think that the "Once Catholic, always Catholic" is the least bit viable ... especially after Mystici Corporis killed it.