You are a "theistic evolutionist", who is struggling with many Biblical concepts that you must distort and misinterpret in order to conform to your false belief. Almost half of Christians accept evolution this way because they either don't understand it, nor do they understand scripture.
It is not the foundation of our religion, but the origins of man are a foundation of who we are and where we came from. If you distort the origins, then who we are and where we came from is up for speculation. Hence Darwin speculated and was wrong. He speculated that we evolved from a primordial goo. Genesis says God created man, plants and animals complete with no evolutionary process -- a finished product. A peacock was always a peacock, a rose, a rose and man always man.
You misunderstand the Jewish style of writing; what appears to be chronological to you, is in fact a typical writing style used where the first chapter is an outline summation and then the second chapter goes back and fills in details. Let us make man in our image ... so God created man (which was Adam at that moment and shortly after Eve).
See you later guess it when you suggest this: "Either Genesis backtracks and inserts more material into the account of the sixth day, or Adam and Eve aren't part of the six day creation."
You are assuming there was a wall. It doesn't say anywhere about a wall. God kicked them out of the Garden and placed Cherubim to guard the garden. "Probably square ..." Another assumption.
Do you read anything about a wall here?
You are right, the Bible doesn't say anything about destroying the Garden or removing it. We don't add to the Bible in this regard, we just discern that the Garden slowly lost it's perfection due to sin. When sin was introduced, it began to change and distort the genetic code, and things began to die - not instantly, but in a cellular, biological way. Diseases, viruses, bacteria all began on that day sin was introduced, thereby distorting what was perfect. So the paradise just got corrupted and eventually turned into what we see today.
You see, the Garden was really the whole planet. It was all beautiful and we still see beauty today, it's just when we look close, we see the defects and of course where there were lush gardens, we see deserts that have expanded. That area, which contained four rivers (two exist today, the Tigris and Euphrates), is in Iraq and now desert.
This is the key point you overlook: What God was guarding was not so much the lush region of lush green land filled with fruits and vegetables that we still have abundantly today, it was the Tree of Life. They ate from the Tree of Knowledge of Good and Evil, so if they had then eaten of the Tree of Life, they would have remained in that sinful state forever - this is what God saved them from -- so He guarded it until whenever.
He would later send a Savior to rectify the problem. So we can assume that eventually that the of Tree of Life withered and died. However in Revelation, we do see a Tree of Life in Rev. 22, so He could have removed it or created a new one.??