Although I agree with much of what you wrote, there is one point that requires some clarification.
The law does not exist because of sin. It's like saying, "because someone killed another human being we have determined after the fact that it is unlawful."
God is all-knowing - He knows the beginning from the end. He did not have to wait and see what might happen before deciding to develop a law and then decide what sin is.
The law that was added involved the animal sacrificial ordinances (this is not easily identified by most). THE law - as in the spiritual legislation from God is spiritual in nature and has always existed as the Apostle Paul rightly pointed out, "The law IS spiritual" (Rom. 7:14b; cf. Mt. 5:20-22; 1Cor. 10:3-4; Jas. 1:25; 2:8).
The law of God defines what sin is,
"Sin is the transgression of the law" (1Jn. 3:4b).
Thus, the law had to exist prior to sin, otherwise, it's not possible to commit sin,
'What shall we say then? Is the law sin? Certainly not! On the contrary, I would not have known sin except through the law. For I would not have known covetousness unless the law had said, “You shall not covet”' (Rom. 7:7).
The Adversary, who was the very first being to commit sin in his rebellion against God, could not have sinned if the Law of God did not yet exist.
I feel like this could be a conversation that could go into any number of directions. So I think I'll simply try and explain where I'm coming from.
In the Reformation the Evangelical fathers articulated three "Uses of the Law" as part of the Dichotomy of Law and Gospel (the radical distinction between Law and Gospel). The Three Uses of the Law are as follows:
1) The Law curbs evil, to command what is right and to forbid what is wrong.
2) The Law is a mirror that exposes our actions as unjust, the Law says "Do this" and we don't do it. The Law in this way reveals us to be sinners, and therefore condemns us as such--as those who do what is unjust rather than what is just. We
don't love our neighbor as we should.
3) The Law provides the regulative principle for how the people of God ought to conduct themselves and live--we are to love our neighbor as ourselves.
The imperative to love our neighbor, to love one another, to love the world of God's creation will never cease--this will be the way of things in the Age to Come, because we actually
will be loving as we ought to love.
But without sin the Law's "power" as a harsh, burdening power won't exist.
God is not going to rule the Age to Come by the imposition of harsh commandments, but by living and dwelling the midst of the world in fullness. We will dwell in perfect love, free and generous without any force.
So if one understands the continuance of God's law in the future age as simply the fullness of love expressed in the harmony of God and His creation, then that's fine.
However if we are imagining the Age to Come to be a kind of theocratic "government" in which laws exist in a top-to-bottom hierarchy--that is significantly problematic as it is contrary to what Christ Himself has taught us about God's way of being king.
God's decree is that we
live and
love.
And therefore, "The sting of death is sin, and the power of sin is the law." (1 Corinthians 15:56), since "It was sin, producing death in me through what is good, in order that sin might be shown to be sin, and through the commandment might become sinful beyond measure." (Romans 7:13)
But without the old Adam, without sin no longer dwelling in our members, what is the Law? The answer I believe is as I've said: That we live and love, enjoying God and all creation together.
-CryptoLutheran