Read the story in context.
3:24 So he drove out the man; and he placed at the east of the garden of Eden Cherubims, and a flaming sword which turned every way, to keep the way of the tree of life.
4:1 And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD.
4:2 And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.
The story goes straight from being driven from the garden to the birth of Cain, with no mention of intervention centuries of births and the construction of cities etc. Your reading doesn't pass the "sophomore test", i.e. composition and narration skills at least equal to that expected in a sophomore English class. Stretch and twist in the spirit of Ruth Bader Ginsburg as much as you like, but you hypothesis does not pass a plain reading of the text.
Like I said, Cain was the 1st one mentioned but that doesn't mean he was born first. We have no idea how long they were in the garden or if they had children before the fall. There's no mention of time at all. After he murdered Abel, Cain was afraid of those other people out there. If he and Able were the 1st, who would those other people have been?
These are things that we aren't told. You're trying to inject info that simply isn't there.
