Yes, you can be. The Church compelled many heretics to do 'x' even though they thought 'x' was heresy. I'm not saying that anyone on either side of this debate is a heretic, i'm just pointing out that "one's conscience" is not the end all, be all. You must have a CORRECTLY FORMED conscience, first of all. And even then, the Church's decision on a matter of faith/liturgy has nothing to do with one's conscience. You either obey or not.
Alright, I should have phrased that differently. To act against a certain conscience is sinful. Whether or not you sinned through vincible ignorance arriving at a certain conscience, to act against what you believe to be morally right is a sin. I don't think a heretic can be presumed to have a inculpable certain conscience after being corrected by the Church because the Church is the sole authority that can teach infallibly on matters of faith and morals. They have been made aware of their heresy, whether they were ignorant or otherwise.
I think Fr Wathen's take on this is that NO ONE can come to any moral certainty about the crisis, except the Church. Ergo, this whole issue remains in the realm of opinion. Much like the status of the new mass' validity...even Fr Wathen said that the Church will have to decide this matter. All any of us can do is make decisions on 'positive doubt'. But no one can claim with 100% certainty that the new mass is invalid.
As far as I know, Father Wathen came to a 100% moral certainty that the Novus Ordo was sacrilegious. He said that the faithful could not attend an NO mass for any reason, even a funeral out of respect for the dead. Genuinely asking, what exactly is the difference between his moral certainty on that and a sedevacantists moral certainty on the status of the papacy? How could he have come to that conclusion without the Church deciding the matter?
Moral certainty on a matter necessarily means that ALL CATHOLICS would have to follow said idea. There's no such thing as "Bob's moral certainty" vs "Jim's moral certainty". That's just the error of subjectivism. In absence of a church decision, it's all opinion.
The point being, there is no way anyone can have OBJECTIVE certainty on the papal issue until the Church decides. Everyone can ascertain that the V2 popes are heretics, but to what degree? At what point (and who decides when?) does a heretic pope lose office? This issue has never been decided. So it's just opinion.
I don't think so. Moral certainty is a personal judgement. I may be morally certain that the NO mass is sacrilegious (in the sense that I would not attend a Latin Mass at a Novus Ordo church due to the desecration of the altar), but an SSPX priest may just think it is "deficient" or "irreverent" and may have no problem saying mass at a NO Church with permission from the local Bishop. What authority is going to decide on the matter for us?
My point is that if you say that a sedevacantist priest cannot omit the name of a conciliar Pope because he is inserting his own opinion into the Mass, you are elevating your own opinion regarding the papacy to a fact. If a sede priest omits the name of the conciliar Pope, he is simply doing what he believes to be right