Those who adhere to a theistic evolutionary viewpoint have reduced the Bible to being nothing more than an allegory or a non-literal work.
Not ture. I'm a theistic evolutionist and I think there is more to the Bible than allegory. It reveals some of the most important and great truths of our world.
A good analogy is the map/territory one.
The real world is the territory and the Bible is the map. There will be some places where the map differs from the territory but it is still a useful tool. It is important to remember that the map is not the territory, itself.
In other words, don't worship the Bible.
If the creation story is not literal then how does one know what is literal (if anything) in Scripture?
You don't.
Do you see the problem?
Yes, but only if your faith is a house of cards.
I don't know that Jesus was a literal person but I have faith that he was. If it had to be 'proven' to me that Jesus was a literal person before I believed it then I would have knowledge, which is the opposite of faith.