1. How many of them been confronted with these alleged errors, and if they have not, are you sure they are "manifest heretics"?
Yes, I am sure, why? A manifest heretic is a heretic who is not an occult heretic. Yes, I am sure that they're manifest heretics, they don't conceal the fact that they adhere to the robber council.
[9] But avoid foolish questions, and genealogies, and contentions, and strivings about the law. For they are unprofitable and vain. [10] A man that is a heretic, after the first and second admonition, avoid: [11] Knowing that he, that is such an one, is subverted, and sinneth, being condemned by his own judgment.
Some read this to mean: If he hasn't been admonished, then he is not a heretic. But that's not what Paul says. The gist of what he says to Titus is: before you avoid a heretic, be charitable and give him a chance by admonishing him. On the other hand, don't spent too much time on him. And you don't need to condemn him (avoiding is sufficient) since he already is condemned by his own judgment.
Heretics may be classified as material vs. formal heretics, and Van Noort says:
It is the more common opinion that public [i.e. manifest], material heretics are likewise excluded from membership. Theological reasoning for this opinion is quite strong: if public material heretics remained members of the Church, the visibility and unity of Christ's Church would perish. If these purely material heretics were considered members of the Catholic Church in the strict sense of the term, how would one ever locate the "Catholic Church"? How would the Church be one body? How would it profess one faith? Where would be its visibility? Where its unity? For these and other reasons we find it difficult to see any intrinsic probability to the opinion which would allow for public heretics, in good faith, remaining members of the Church.
sourceAlso, see my signature. Father of the Church St. Jerome, and Doctor of the Church St Robert support the more common opinion.
2. Are you bound to accept DH, and if so, at what level (faith, piety, etc.)
None can be bound to accept heresy, at whichever level. This question is not pertinent. Heretics do not need to bind anyone to be heretics. Rather, heretics cannot bind anyone to swallow their poison.
3. Are you certain that DH is contrary to QC, and if so, what level of certainty?
I am as certain as a man can be with respect to such questions. And I am obliged to follow my conscience whether I'm right or I'm wrong (see St. Thomas Aquinas).
Why would the Apostle ask us to reject a false gospel, if we shouldn't accept our own judgment based on received Church teaching? Why would the Council of Trent say
As, however, it is not enough to speak the truth without discovering and refuting error, it has pleased the Holy Synod to subjoin the following canons, so that all, now knowing the Catholic doctrine, may also understand what heresies they have to beware against and avoid.
sourceif we shouldn't accept our own judgment based on Church doctrine?
The Old Catholics say Vatican I was a "robber council" and the Catholic church hierarchy has adhered to that error and lost all authority ever since. So how is sedevacantism not another version of Old Catholicism?
Vatican I doesn't teach heresy.