CathInfo.com

1. How many of them been confronted with these alleged errors, and if they have not, are you sure they are "manifest heretics"?

[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.

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.

2. Are you bound to accept DH, and if so, at what level (faith, piety, etc.)

3. Are you certain that DH is contrary to QC, and if so, what level of certainty?

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.

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?

This is based on Van Noort's IMO defective understanding of material vs. formal heresy.  There was a big semantic debate among theologians as to whether the term "material heretic" should even exist.  Van Noort's use the term implies bad will or insincerity that can only be known in the internal forum.  In order to be even a material heretic, however, pertinacity is required.  You can't just consider someone to be outside the Church because they blurt out a heretical proposition.  They could have just mis-spoken or are just plain ignorant.

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.