I'm afraid that there's long been confusion about the term "formal heresy". No, it isn't the equivalent of formal / official condemnation by the Church. Nor is it an internal forum consideration that's tantamount to being "insincere". It's often said that those who are "sincere" are not formal heretics ... as a part of the over-200-year-old campaign to gut EENS dogma.
Where it comes to supernatural faith, there are two aspects, the material and the formal. Material refers to WHAT is believed, and the formal to WHY it's believed. So the various defined dogmas of the faith are the matter or the material faith. But these beliefs must be held by a supernatural motive, namely, that of submission in intellect and will to the authority of God revealing through the authority of the Catholic Church. It is theoretically possible that someone might believe every single teaching of the Church but if they developed these beliefs by their own personal interpretation of the Bible rather than on the authority of the Church's teaching, then they are still formal heretics. This is also why it's said that if you deny one dogma, you deny them all. Why?, because you're implicitly rejecting the authority behind that one dogma, which is the same authority behind all dogmas, so you FORMALLY reject all dogma because you reject that which provides the formal motive of faith, the Church's teaching authority. Formal heresy has nothing to do with sincerity (or, rather, the lack thereof). So, for instance, you could have someone who is raised Protestant and is perfectly sincere in accepting the Protestant beliefs, but because he doesn't believe what he believes based on the authority of the Church, he's a formal heretic. Material heretics are those who are only mistaken about some details regarding WHAT the Church teaches, and, as St. Augustine indicates, the litmus test for a material heretics is that, as soon as you explain to them, "No, that's not what the Church teaches, but it's this." the individual immediately retracts it and then accepts the correct teaching.
When individuals attempt to twist the notion of "formal" heresy into "sincerity," they also make it something that's therefore a matter of the internal forum, which can be discerned only by God, and not even by the Church. Those who demand proof of formal heresy for the V2 Anti-popes deliberately set up a criterion that can never be met, since according to their understanding and definition of formal heresy, it can never be known in the external forum. This is a ruse used by R&R.
But St. Robert Bellarmine teaches about MANIFEST (pertinacious public) heresy and not FORMAL. This criterion of "formal" heresy, as falsely re-defined into "sincerity" is nothing but a ruse. St. Robert Bellarmine explicitly rejects the idea that we are required to or can read hearts in order to ascertain public heresy, but states that we just them heretics according to their works, i.e. their outward actions.
At the end of the day, though, this doesn't matter. We know that the V2 Papal claimants do not (at least not freely) exercise legitimate papal authority, because the legitimate exercise of papal authority is protected by the Holy Ghost from destroying the Church as badly as has been seen done by the V2 Antipopes. Perhaps the only possible out from sedevacantism would be to posit that the V2 papal claimants have been blackmailed or otherwise coerced so that their authoritative decisions were not free. Certainly it's not impossible in the case of a Montini, who very likely was a practicing sodomite and had fallen into the Communist honey trap. Those who insist that legitimate papal authority can corrupt the Magisterium and the Public Worship of the Church (the Mass), they're in grave danger of losing the Catholic faith and are flirting with Old Catholicism, and in some cases can barely be distinguished therefrom.