Recusant Sede,
You, being a fervent Sedevacantist, (as evidenced by your choice of screen name -- your very identity here) represent the minority "tolerated" opinion on CathInfo. This is not a sedevacantist board; it is a Traditional Catholic board which doesn't EXCLUDE sedevacantists. There is a difference.
There's nothing I can say that you will agree with on this issue. We fundamentally disagree on the state of the Papacy.
But to answer your question, YES, a Catholic can know with moral certainty when something is just WRONG and when something is fine. It's called the sensus Catholicus, or Catholic sense. Saying we can't make a prudential judgment call on something like this is akin to saying that human beings can't attain to the truth, like some kind of relativist philosophers would have us believe.
Since YOU brought it up, I will give you my opinion: I personally think it's crazy when some Sedevacantists claim that we can't go ahead with any kind of moral certainty and disobey the pope when he's severely destroying the Church -- we have to deny his papacy; that's the only answer.
But any further discussion of the Papacy needs to be taken to the Crisis in the Church subforum.