I'm not a fan of the subjective vs. objective distinction, often also laid out as the "internal forum" vs. "external forum" distinction.  That confuses the matter.

I refer to FORMAL vs. MATERIAL.  Formal does not mean "subjectively" ... as it has been spun (with the rise and growth of subjectivism, which is the core thinking behind V2 and Amoris Laetitia, etc.).  Formal refers to the fact that you effectively (at least implicitly) reject the rule of faith, the teaching authority of the Church.  This is why it is said that if you (pertinaciously) reject one core teaching of the Church, then you reject them all.  Why?  Because you reject the authority that's behind all teaching.  So in order to be a FORMAL heretic, one has to reject the teaching authority of the Church, at least implicitly.  That is why the priest in that thread joining Orthodox is a FORMAL heretic.  He has rejected the teaching authority of the Church.  It matters nothing whether he's sincere or insincere.  That is a matter of the internal forum that God alone can judge.  If the definition of FORMAL heresy pertains to matters of the internal forum, then NO ONE can ever be denounced as a heretic.  This was clearly articulated by St. Robert Bellarmine, and it's why he emphasizes MANIFEST heresy.

NO TRADITIONAL CATHOLIC is a FORMAL HERETIC (well, there might be one or two lurking somewhere).  I know of no Traditional Catholic who rejects the teaching authority of the Church.

Those who take some kind of theological position by reasoning from the teaching of the Church and who claim that their position is consistent with Church teaching are prima facie NOT formal heretics, since the fact that it matters to them that their position reconciles with Church teachings clearly indicates that they CARE about being consistent with Church teaching.

While their position might be wrong and might in fact actually contradict Catholic teaching upon analysis, i.e. that kind of objective error does not entail formal heresy.

In fact, many theologians argue that the term "heresy" is incorrectly used in the expression "material heretic".  Heresy (from the Greek haeresis) indicates "clinging to" or "adhering to" some error, so reasoning from the etymology of the term, some theologians say that merely material heresy is a contradiction of terms.

Basically, though, the term has been applied (and some theologians did persist in using the term "material heresy" against those pushing the etymological sense) to those who reject a teaching of the Church that has the note of dogma.  So, in other words, if you commit some error against Catholic doctrine that does not have the note of de fide, you could be committing a grave sin against faith, but you would not be a heretic.

So some theologians use "heresy" as a reference to the theological note of the doctrine/dogma being denied, and others use it only to refer to pertinacious adherence to said teaching.  This has been a major source of confusion, and I read a treatment of this problem in a pre-V2 theologian.

So throwing the term "heresy" around is inherently confusing.  In fact, though, the Church has condemned various errors with the note of "heresy", and so the Church has used the term to refer to the theological note rather than to pertinacity, despite the arguments of the etymoloists.

Perhaps a compromise position would be that the term "heresy" refers to a denial of a teaching that has the theological note of de fide, while the term "heretic" should only be applied to those who effectively reject the teaching authority of the Church.

With this, you could say "your position is heretical" but not "you are a heretic."  So, you could say, "Sedevacantism is heretical." but not "Sedevacantists are heretics."  I think this would be a good compromise between the two schools of thought and might lift some of the confusion.

DL,
I believe RT is saying you need to add a 3rd condition, which would be “acceptance of the visible Church” or something along those lines.  RT did not take for granted that your “professing the Catholic Faith” included being subject to the papacy because he feels (oddly) that SVs have rejected the papacy and also the visible Church.  I think that’s the point of his long, rambling post - acceptance of the visible church. 

I think RT is making the argument that SVs fall into the above category.  He's basically equating SVs with schism.

Did you actually miss this?

Sedevacantists are heretics for defecting from the Church and the Faith and joining heretical sects.