I suggest you NOT paraphrase. I also suggest you not take one provision out of context.
What the Catechism actually says is:
1790 A human being must always obey the certain judgment of his conscience. If he were deliberately to act against it, he would condemn himself. Yet it can happen that moral conscience remains in ignorance and makes erroneous judgments about acts to be performed or already committed.
So . . . answering you second set of questions first, it is important to note the term “CERTAIN”. If you aren’t “sure”, the provision does not apply.
The Catechism also says:
1781 Conscience enables one to assume *responsibility *for the acts performed. If man commits evil, the just judgment of conscience can remain within him as the witness to the universal truth of the good, at the same time as the evil of his particular choice. The verdict of the judgment of conscience remains a pledge of hope and mercy. In attesting to the fault committed, it calls to mind the forgiveness that must be asked, the good that must still be practiced, and the virtue that must be constantly cultivated with the grace of God:
So . . . answering your first question next, if a person repents of committing an evil against the just judgment of his certain conscience, he is forgiven if he sincerely seeks the forgiveness.
NO SIN IS UNFORGIVABLE. NONE ZERO ZIP.
. . if you only ask.
Do you happen to struggle with scruples?