Secondly can we argue that it is to be seen as the eternal law of God? It didn't exist before creation and it certainly won't exist in eternity where there is only the glory of God and no need for a sun and no concept of days. In fact, we know that eternity will be one long Sabbath but it will have nothing in common with the one found in Exodus 20.
Perhaps part of confusion lies in looking at the Fourth Commandment as looking backward to a supposed creation ordinance instead of future to a greater fulfillment. After the six days of creation God ceased from his labors. Adam and Eve were created to enjoy this rest by enjoying all that he had made for them and to be provided for by trusting in his providence. They fell by saying that they would instead do things their way and not trust in the Lord. They were saying that they no longer would be willing to rest in God but would do their own work.
It is interesting that the primary curse put on Adam besides his fallen nature is that all of the sudden he had to labor in a way that was mostly futile, Gen 3:19 "By the sweat of your face you shall eat bread, till you return to the ground, for out of it you were taken; for you are dust, and to dust you shall return." In other words, man in his fallen state has no rest and his labor has no better end than death; it is futile. Unfallen man had to labor but it would always result in fruit unto the Lord; it was joyful, fulfilling and satisfying. All that changed at the fall.
And so immediately God starts to make promises that he is going to restore the rest that Adam rejected. If this is in some way summarizes what happened in Genesis then it seems that the Sabbath laws could be given to look forward instead of backwards. After all Jesus says more than once that the law and everything in the OT taught of him and can only be fulfilled in him, Mat 5:17-18 "Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. For truly, I say to you, until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished."
The theme of a coming rest is seen throughout the Bible. In Matthew 11 he alludes to it pretty clearly, Mat 11:28 "Come to me, all who labor and are heavy laden, and I will give you rest. Mat 11:29 Take my yoke upon you, and learn from me, for I am gentle and lowly in heart, and you will find rest for your souls. Mat 11:30 For my yoke is easy, and my burden is light." Notice that this rest doesn't mean that there won't be plenty to do. We still are under a yoke and have a burden but his service is joyful and fulfilling and accomplished God's glory and our good.
If we read through Hebrews 4 the writer makes is quite clear that there is a rest waiting for all who come by faith to the cross of Christ. He compares it to the rest of God at creation. So in the first creation God did a "good" work that provided a good rest for man but he sinned it away and has been feverishly working ever since to correct but to no avail because sinful man can't fix the problem by his own works. In Christ, God has done another work, a perfect and final work and all who enter into this rest will have rest for their souls and can never be cast out as our first parents were.
The Sabbath that the Bible always looked forward to and the only one we need to be concerned about is the rest through faith in the finished work of Christ. The Sabbath as found in Exodus 20 was never meant to be some eternal binding principle but looked forward to the cross and it is in obedience to the gospel that we keep "Sabbath" as NT Christians.