My question: How then do you determine what is literal truth and what is parable? Is that not a slippery slope? Can the entire Bible just be a parable under this logic?
You are correct, it is a very slippery slope. Most of the allegorical examples are obvious and have been seen as such since they were first spoken.
John 15:5 I am the Vine; you are the branches.
Psalm 22:14 I am poured out like water, and all my bones are out of joint: my heart is like wax; it is melted in the midst of my bowels.
Matthew 13:44 Again, the kingdom of heaven is like unto treasure hid in a field; the which when a man hath found, he hideth, and for joy thereof goeth and selleth all that he hath, and buyeth that field.
Most Christians have been cowed by the intensity of secular "science" that purports to prove the Bible wrong, particularly about creation. Since very few people actually understand science, they are bullied by the consensus of those who claim expertise.
Those Christians you have met that "admit" the Bible is full of errors and that it cannot be relied upon, have been crippled by the god of this world. They should not be seen as a source for accurate understanding, but more pitied for the injuries they have incurred by straying so far from their Lord.
In fact there are only two ways to look at our existence. We are either an accident (nothing existed, then it blew up and became everything and then became people), or we were created.
If you take the so-called Bible "errors" one by one a go through the slow, boring process of refuting each one, you find that the charges against the Bible all fade away. Most people do not have the endurance or ability for such an undertaking and subsequently choose a position based on what most other people will object to the least.
The key to the Bible is understanding truth and having wisdom. Truth and wisdom are found in God. Since the secularists have cut themselves off from God, it should not be a surprise that they tenaciously cling to their "religion" (secularism can be seen as the worship of man in general and self in particular).
1 Corinthians 1:18 For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God.
1 Corinthians 2:14 But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned.
Romans 1:22 Professing themselves to be wise, they became fools,
The key to the Bible is truth. The key to truth is an absolute God. Only those seeking truth find God. God helps those seeking truth because they are drawn to Him.