It's really not a matter of spectrum. Hermeneutics and exegesis are tools to get at what the author really intended to say. Metaphors and simile's, that take the form of common idiom in the authors day, have to be sharply distinguished from metaphor or idiom that is intended by the author to be part of his instruction or argument. Confusing the two is not a matter of degree but a pass/fail issue of bad scholarship.
The word doctrine is largely misused by the popular piety and even by leadership in some faiths. Doctrine is a set of rules for interpretation that arise from deep scholarship. Doctrines are the structure of a critical perspective used by a given student. These rules are arrived at by careful study that entails trying to put yourself into the mindset of the original audience of a given work and construct coherent tools for sharing that understanding with others.
After you have gotten at what the author is intentionally communicating then the questions of whether this is allegorical or literally truth come into the equation not before. This is an entirely different question. The arguments between scholars over these issues do come down to literality vs allegory. Fundamentalists are only one group among many who believe that the intended arguments, historical accounts and didactic instruction are literally factual and true because of that. Comparing all these views to misinterpreting an idiom is polemical.
It's tantamount to saying a divinity student who has immersed himself in biblical language, archeology, sociology and hermeneutics is just some uneducated buffoon who picked up a Bible and decided that he was reading a science text written in English 2 years ago. Even your mocking argument about the sheep writing poetry doesn't hold up.
A hyper-literal mind like you propose would see God herd the MAN David as if he were a sheep and God was his shepherd. Even changing David into a sheep requires some awareness of the oddity of the phrasing and the oblique descriptions. In fact the word sheep (hershel) never appears in psalm 23 so it would take a mind with at least the subtlety to understand the allusion, in order to see David as a sheep.
That leads us back to the issue of the student. A serious student capable of deciphering the allusion is also going to see the idiom. This illustrates my point that assuming differences in interpretation are as simple and drawing a line where you are going to stop taking things literally ignores the basics of language. If I said, "You are jumping from the frying pan into the fire with that logic," taking me literally would entail considering if I mean you are literally going to get into trouble.
Only a small child would think I really meant a pan or a flame. It's not a matter of degree but of cognition. By suggesting the Bible scholar would or even COULD see pans and flames in this construction is suggesting that 90 units of grad school makes one into a child.
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note on edits.
changed make or break to pass/fail in the first paragraph with the caveat that I don't literally mean that the reader on the scholar are in my class. I will not be grading posts. Please understand this is idiom.