I think you are wrong about losing narrative coherence. What could be more incoherent than the references to Cain and the mark he receives so that he won’t be killed by people who only know he is a murderer. By whom? Why they should they kill him? If God can tell them not to kill “the man who has the mark” … he can just as easily tell them not to kill Cain. If Cain is the only person out there… nobody is going to mistake who he is. But is he the only one? He builds a city? For whom? Who are all these people living in this city?
And then there is the issue of trying to build coherence between the different sequences and aspects behind the two very different stories of creation.
Finally, I think you are a little too eager to repeat Galileo’s quote: “The Bible is about how we go to heaven, not how the heavens go.” Firstly, Galileo was referring to the New Testament.
Secondly, you don’t find it at all odd that we read about a man giving up his birthright over a bowl of pottage … but the closest thing we have to Biblical awareness of millions of years of Earth’s history are the two different stories in Genesis?
While you seem to expect some kind of interpretive purity (by making the two stories about the same 2 humans), you seem perfectly at ease regarding ignoring the contents of both stories and applying them to Old Earth history.
If you endorse Old Earth history, I would expect just a little more eagerness to examine the parts of the Bible that just might support that bone of contention.