It's not a matter of understanding metaphor. I understand metaphors just fine. I'm not arguing that the Bible has to be interpreted literally rather than metaphorically. The problem is adding stuff to Genesis was wasn't put there, parable or no. If you want to say that Adam was supposed to understand that God meant "spiritual" death when that was never mentioned, then any part of the Bible can mean anything you want it to. Then it really would become an "unconnected set of words from beginning to end".
That is a dilemma, as to interpreting one's own understanding into the Bible, as opposed to taking away from it a harvest of understanding.
If you sow sparingly you will reap sparingly. So I try to stay open and keep my mind fertile.
I just see the stories in Genesis, as the Beginning. And we can trace back to that Beginning. If Adam were a spiritual man, made in the image of God, as Genesis states, then Adam would understand what God meant, and I don't have to project anything into the story.