Honest Questions about the Seventh-day Sabbath

Wasn’t the seventh-day Sabbath given at Sinai for the Jews? 

“Thou shalt not kill” was also given at Sinai. Does that mean that Cain did not sin by killing his brother? Did God just introduce some commandments early and reserve others to be introduced along with the Sabbath at Sinai? Hardly, for the Bible states that if you break one, you break them all. James 2:10. Thus, God shows that the law is one complete whole and not ten stand-alone rules independent of each other. 

Before Sinai, Pharaoh had a knowledge that the worship of Jehovah included a day of rest. When Moses relayed God’s message to Pharaoh, “Thus saith the Lord God of Israel, Let My people go, that they may hold a feast unto Me in the wilderness,” Pharaoh responded harshly: “Ye make them rest from their burdens.” Ex. 5:1, 5. While this is a