I wish I had more time to give a thoughtful reply before work this morning, but alas, perhaps a simple reply is better than none at all.
I still can't make heads or tails of your argument because it still seems based upon a faulty premise. To state it succinctly I can't get around you saying that God's covenant (I used the word because you want to use it, not because it is Biblically described that way) with Adam was fulfilled and completed with Adam. Yet Romans 5 makes it clear that because of Adam death and sin reign over the human race - apart from Christ that is. And we know that death and sin won't finally actually once and for all be destroyed until the end of the age.
How can you say that their contract (Adam and Eve's) wasn't a contract for the human race if the terms or consequences are for the whole human race? If what you are saying is true and the contract ended with them then their progeny would not have experienced sin or death.
Again, can't figure out why it is so important for you to characterize what happened with Adam the way you do and it certainly doesn't seem biblical to me.
When Adam interacted with God, he lived in a perfect world. No sickness, no disease, no death. He had an understanding as to behaviour, with consequences and those consequences included both positive and negative possibilities. It is not elaborated upon by the scriptures but we know some things without being told. For example, Adam was told that if he ate of the fruit of the tree of knowledge of good and evil, he will die. THIS includes a certain understanding that if he does NOT so eat, he will live. THAT makes a situation plausible known as an agreement. Not a scriptural quote, but a scriptural teaching.
Adam ate, and died. Terms of agreement and understanding fulfilled. Adam sent out of the garden. Earth cursed. Ends Adam's part in the scheme of things. However, the "Consequences" of Adam's action are still with us today. But NOT in the same world as that of Adam. Remember, he began life in a perfect world.
God cursed the earth, so that by the time the next generation knew anything at all, it was from the standpoint of living in a sincursed world. There is no record of any transaction between them and God other than the requirements of sacrifrice pleasing to God. Cain and Able had to sort that out in their own way. Able's sacrifice pleased God, Cain's did not; from which we can draw a different set of conclusions. God had to have communicated to them what would please him as to the type of sacrifice, and the type of comodity they could use. One is from a flock of living animal, having blood coursing through its veins, wherein life flows. {Remember, "the life is in the blood."} Cain offered the fruit of his own labor, of vegetables having no life-carrying blood. His sacrifice was contrary to God's pleasure, and his obvious instruction. Again, consequences, and promises for certain kinds of behaviour as respects sacrifice and obedience.
And that was the only interaction we know of with only one exception, Enoch, who walked with God and was not for God took him, because he pleased God. There is no way Enoch could have pleased God without God's communication to men, and Enoch's pursuit of righteousness in line with that communication. Again, not a scripture quote, but a scriptural teaching.
Finally came a time in which God communicated again, his displeasure with the race of men, by flooding the earth with water. Once more, we can develope an understanding about what those men knew by what God did in reaction to their efforts. When God destroyed the earth because of man's behaviour, there was no innocent man walking the earth. ALL stood guilty before God. "Every thought of man's heart was only evil continually." THAT is quite an indictment. But it tells us they were not kept in ignorance of God's will, but rather chose to go their own way. Remember, Enoch lived up during some of that same time and pleased God, so it was a matter of choice; how Enoch reacted to God's communication compared with how all other men reacted to that same communication.
But remember also, it was not the same world as that of Adam. Nor was it the same set of instructions as that of Adam, nor even of Cain and Able.
Time, world situation, people involved, promises, consequences, blessings, curses, all different for each group of men the Bible presents to us for our learning. And remember also that the old testament was designed to lead the Jews to Christ. All of these stories are part of that old covenant with Abraham, and part of those lessons to lead them to Christ.
It constituted a build up of different requirements as to what pleased God, who accomplished that, and results of effort extended.
There are some results from those days that still effect us today without our being part of those agreements and understandings. Example - cursed earth; we live with it. Example -Rainbow symbol of covenant-promise; we live with it. Example - prophetic utteraqnces concerning Messiah and salvation; we live with it. Example - prophecies of wars, rumors of war, pestilence, etc,; we live with it.
All of the covenants with God had results that affected those within its purview, AND also had results that extended long after those men died. Romans 5 is proof of that. The tale of the flood is further proof. The tale of the allegory between Sarai and Hagar, "which are two covenants" tells us yet again of things that carry results way beyond the time of those involved.
I don't know if this clears it up or makes it worse. I do know that all the covenants are connected, and consequences of one are still effective in another. Patience.