Vatican I doesn't teach heresy.
The Old Catholics think it does and make essentially the same argument as sedevacantists.
Well, just like you and me, the Old Catholics have an obligation to follow their conscience, whether erring or not.
You have taken the huge step of separating from the pope.
Thousands of putative bishops have taken the huge step to teach condemned heresy, and I have taken the small step to reject heresy and avoid heretics.
You have taken the huge step of separating from the pope. If you're going to take a step like that, I would expect to see evidence of substantial study and some introspection.
I am just answering your questions and not authoring a dissertation on the topic. And so far, you haven't asked for one.
But I agree, I too would have expected that e.g. Döllinger would have presented at least a booklet or something, providing a concise defense of their rejection of the General Vatican Council.
I, on the other hand, feel comfortable with sorting things out for myself (not without reading all sorts of literature on the topic) and presenting my conclusions e.g. here on CI, to harvest objections, some of which allow me to rethink and improve my points of view.
Yet you don't seem to make distinctions about different meanings of "manifest heretic" according to different theologians.
There is no debate between theologians about the manifest vs. occult heretic distinction. There may be minor differences in the way they explain the terms, but these are irrelevant with respect to the false shepherds of the conciliar sect.
You haven't even established that V2 taught something contrary to QC. Theologians have published a lot on DH and QC including some detailed explanations of how they are not contrary. Have you read them all? Along with a wide range of Fathers and other theologians?
DH is against common sense. DH claims that there is a God-given right to act against God-given law. The basic principle of natural right is: Do what's good, omit what's bad. Yet DH claims that there is a natural right to do what's bad (worship idols). One doesn't even need the
ex cathedra condemnation of QC to see that that's a false gospel. The Church never teaches anything against common sense.
I've debated this topic with several Novus Ordites for years, and they presented quite some literature. All desperate attempts to show that what must not be, cannot be.