It is a sin for someone to commit murder regardless of whether or not they want to follow the law against committing murder, they should understand what it means to commit murder regardless of whether or not they want to follow that law, and the same goes for God's other laws.
It would be unjust to enforce a penalty that Jesus has already paid. Still, the fact that God considers breaking the Sabbath to be worthy of the death penalty and the fact that Jesus gave himself to pay that penalty should make us want to go and sin no more.
Yes.
In Matthew 4:15-23, Jesus began his ministry with the Gospel message to repent for the Kingdom of God is at hand, which was a light to the nations, and the Mosaic Law was how his audience knew what sin is (Romans 3:20), so repenting from our disobedience to it is a central part the Gospel of the Kingdom, which is in accordance with Jesus being sent in fulfillment of the promise to bless us by turning us from our wickedness, which is the Gospel that was made known in advance to Abraham in accordance with the promise (Galatians 3:8), which he spread to Gentiles in Haran in accordance with the promise (Genesis 12:1-5)
In Genesis 18:19, God knew Abraham that he would teach his children and those of his household to walk in God's way by doing righteousness and justice that the Lord may bring to him all that He has promised. In Genesis 26:4-5, God will multiply Abraham's children as the stars in the heaven, to his children He will given all of these lands, and through his children all of the nations of the earth will be blessed because he heard God's voice and kept His charge, His commandments, His statutes, and His laws. In Deuteronomy 30:16, if the children of Abraham will love God with all of their heart by obeying His commandments, statutes, and laws, then they will live and multiply and God will bless them in the land that they go to possess. So the promise was made to Abraham and brought about because he walked in God's way in obedience to His law, he taught his children and those of his household to do that in accordance with spreading the Gospel of the Kingdom that was made known in advance to him, and because his children did that in obedience to the Mosaic Law. In John 8:39, Jesus connected the works that they should be doing with the works that Abraham did, and the works that they should be doing were in obedience to the Mosaic Law. Moreover, there are many verses that describe the Mosaic Law as being God's way, so both Abraham and Moses walked in God's way in obedience to His law and taught others to do that.
Not offering children to Molech isn't mentioned at all in the NT, but that doesn't mean that we are free to do that. The vast majority of people that Jesus interacted with were Jews who were keeping the Sabbath holy, so it is not like we should expect him to go around telling everyone that they need to be keeping it, so it brought up an number of times. In Colossians 2:16-23, the Colossians were keeping God's holy days in obedience to His commands, they were being judged by pagans who were promoting human precepts and teachings, self-made religion, asceticism, and severity to the body, and Paul was encouraging them not to let any man judge them for keeping God's holy days. So it is ironic when people try to use that verse to justify their refusal to keep God's holy days.
What reason do you have for thinking that the Law of the Spirit of Life in Christ Jesus is something other than or contrary to what the Father commanded to Moses?
Do not mistake God's patience for permission.
The way to have a relationship with Jesus is not by rejecting God's instructions for how to do that. The command to keep the Sabbath holy is not the command to have a relationship with God one day week.