I thought it was obvious.
Chapter One ends without a garden event
no names
no law
no tree.....no test
Adam is PLACED in the garden
he had an existence prior
eve is a clone.....yes she is
no navel....not born of woman
and then after the test.....Cain finds a wife
he builds a city
seems like there were other people wandering
in that land of Nod
There are several problems I see in your proposed scenario....
1) No other humans could exist before Eve, since Adam said she had to "become the mother of everyone living". (Genesis 3:20)
2) All humans had to descend from just these two in order to make Romans 5:12, true.
3) Jesus was said to be the "last Adam" because he bought back what Adam lost for his children....perfect, sinless human life. His sacrifice balanced the scales of God's perfect justice...."a life for a life". No descendant of Adam could do the job, because all were now plagued by the stain of sin. (1 Corinthians 15:22; 45)
4) If humans had lived before Eve, then they would not have been subject to sin and therefore would not have received the penalty of death. Only sin produces death.
Apparently Adam was placed in the garden that God had planted as a 'blueprint' of what the whole earth would ultimately become as they populated the planet and extended the boundaries of their paradise home. There he was educated by his God and he observed and named all the animals. He was alone in this situation for an undisclosed period so the final creation on the 6th day was a delight to him...at last "bone of his bone and flesh of his flesh".
There was work to do and God gave them a wonderful start with everything they could possibly need.....but free will had to be harnessed and all had to learn where their own boundaries were so as not to impinge on the free will of others. With the interference of a rebel spirit, things took a turn for the worst. This one abused his free will and thus began an object lesson that would affect not only humankind, but all of God's spirit sons as well. A time a great testing began.
Since there are no time frames to establish how old Cain and Abel were at the time of Abel's murder, the Genesis account mentions "sons and daughters" born to Adam and Eve. Since females are not often named in the scriptures, (e,g. Noah's wife and daughter's in law) even though their presence may have been an important part of the story, it was the lineage of sons that was mentioned in the genealogies. It is very possible that daughters were born to Adam and his wife and that time elapsed enough for daughters to be interspersed between the sons.
Cain and his brother were old enough to have established themselves as distinct personalities before God by the time of the first murder. Their firstborn was not a good man and God's warning to Cain was interesting.....Cain had to alter his course of his own free will....but he allowed jealousy to overtake him to the point of pre-meditated murder.
God only spared him because there was so little numbers of humans to "fill the earth" as he instructed.
Cain's wife could well have been an unnamed sister, since God sent him away to populate a place that was far away from where the rest of the human race would come through Seth. There were no laws on incest at that time....and no way to populate the earth if there was.
Our lives are lived with free will, and God will not alter our actions as we build a reputation with him. We reap what we sow, but all are shackled by Adam's sin. He alone could have changed the course of human history, but he abused his free will and we are all caught in the fallout. Jesus has redeemed us by "becoming the ultimate sacrifice".... a perfect life offered in exchange for the perfect life that Adam denied us.
Until I really studied the Bible, none of this ever made sense to me. IMO, your scenario removes the reason for Christ's sacrifice.