OK, so let’s put it in terms of what would have been understood 2000 years ago. You commit a crime, are caught and brought before the King. You are guilty. You know you are guilty. You admit your guilt to the King. After all this, the King completely forgives you. What sentence do you have to pay if the King has declared you forgiven? If the King has declared you to be forgiven, who has the authority to impose a punishment upon you?
Your second point is moot since you have been forgiven by the King. Romans 6:23 reads, “For the wages of sin is death. But the grace of God, life everlasting, in Christ Jesus our Lord.” I see no distinction of sins (cardinal or venial). Luke 12:47-48 says, “And that servant who knew the will of his lord, and prepared not himself, and did not according to his will, shall be beaten with many stripes. But he that knew not, and did things worthy of stripes, shall be beaten with few stripes. And unto whomsoever much is given, of him much shall be required: and to whom they have committed much, of him they will demand the more.”, which clearly demonstrates the way God views sin (with or without knowledge of what is right). For example, God told Moses that you cannot marry you sister (or half sister), yet Abraham had married his half-sister, Sarah. This happened before the law was given, so there was no knowledge on Abraham’s part that this was sin. Likewise, God instructed Moses that a man may not marry a woman and her sister while they are both alive, yet we know that Jacob had married Leah and Rachel who were sisters and bore to him 12 sons (with some help from their “handmaids”). Again, Jacob didn’t know it was sin because God hadn’t given the law yet. God even made provision in His Law for sin offerings for unknown sin (see Lev 5:17-19).