If Genesis was written by God, but allegorical, it would be symbolic for the truth - a simplified version for us to understand, presumably?... there wouldn't be any actual mistakes, right? As it is, there are mistakes in Genesis I - numerous ones where things are made out of order: Livestock being made before Man, for example... birds (on Day 5) before wild animals and creatures that crawl (Day 6) - when clearly birds were a later arrival... Seed bearing plants and trees that bear fruit, vegetation and creation of the land and sea on Earth (on Day 3) before the Sun, Moon and Stars (on Day 4) - clearly also out of order... day and night (on Day 1) before there was a Sun or stars (Day 4) - also out of order. There's more, but you get the idea. God making it simple for us is one thing... but there would no reason to insert errors into it that would be so simply be correctable (and no harder for Moses or whoever to understand). Clearly Genesis cannot be the exact words of God even if it was only allegorical. (unless God is fallible and sloppy... which, he can't be if we are to trust other parts of the bible) So, the only reasonable conclusion is that it was written by a man - based on best guesses. Further, the differences between Genesis I and II support this, obviously. So, we have a collection of books (the Bible) carefully gathered and selected for truthfulness that includes untruth - fiction. I could also get into how other parts of the Bible are internally contradictory (such as between the synoptic gospels and John, for example) where we have no other scientific or historical records to compare them to and we'd find out that again the Bible contains fiction (whether intended or not) written by man (because God wouldn't do that).
Once you show that there is error (not just simplification, but error) in the Bible, the whole inerrancy argument goes out the window. And once you show the document to be untrustworthy, how are you to believe without doubt any other parts in it that have not been verified by external sources? You can believe what you want (or feel emotionally to be correct) - but what you believe, as far as backup - as far as the Bible, is based on a foundation of air (of error). The Bible is an unreliable source. I'm not sure how the Bible can have "binding authority" when it is shown to be unreliable. That is not to say that nothing in it is true... but then you are reduced to picking and choosing.
Back to Genesis and Creationism... Once the story has been shown to be incorrect and not written by God, what gives us reason to believe any part of it without outside verification? We might be tempted to keep the "Let there be light" part about how God created the universe... but what reason are we basing that on? We have already realized that the document is unreliable... so, we are actually in the position of the writer of Genesis... we are choosing to believe what sounds right to us... with no authority. Our views are bound to be flawed - just as the views of the writer of Genesis was... ...and the "Let there be Light" part was the biggest hand wave of them all... it contains no history, no eye-witness account, and, based on the proven fiction of Genesis, is not the direct word of God. So really, all we are left with, is the faith or feeling of the writer.
As a philosophical concept, I can understand and respect the thought that "thought preceded substance"... no proof either way. But I don't think that you can go to the Bible as a support (due to its unreliability) - you must arrive there (or wherever) on your own.