You are spreading false information in suggesting that Scripture is allegorical as if the literal sense does not take precedence.
Scripture must always be interpreted in the literal unless proven otherwise.
There is a bit of confusion here, possibly due to terminology. When one speaks of allegory that means things like a spiritualized meaning. For example, we are required to believe there was a literal, historical Adam and Eve. We cannot say that Adam and Eve are merely a symbol representing the human condition. It is permitted to say that there are multiple levels of meaning. So, in the example of Adam and Eve, they both really existed in history and are spiritual symbols.
This is a different issue from figures of speech. When we see a phrase like "God saved the people with a mighty arm" we should not take that literally and claim that God has a physical body. There is no requirement to take figures of speech literally and it can be an error to do so. In traditional Catholic (spherical earth) geocentrism and more recent understandings of spherical earth, Catholics understood Scripture that uses flat earth imagery to be using figures of speech.
There is usually no requirement to prove that something is a figure of speech. Normally it is something that is obvious to reason. The majority of Catholics throughout history, including Saints and Doctors of the Church, thought that it was obvious to reason that flat earth language in Scripture was a figure of speech.