They do assume that the narrative portion is the addition. Other mainstream scholars have the reverse — the poetic dialogues were written to flesh out an already-existing narrative.

Either way, it doesn’t necessarily undermine scripture’s authority. The historical books indicate sources used in compiling what we now have. Does that undermine the authority of scripture? How God worked through people’s actions in writing, compiling, and editing is irrelevant to whether God did do so. The book doesn’t tell us the who or the how of its origins.

Also, I think you’re a little quick to dismiss the careful study of scholars simply because they don’t believe in the authority of scripture. There’s much of value in mainstream wok on historical context, languages, archeology, etc., though it takes care to sift the wheat from the chaff.

I appreciate your comment and I do want to clarify that I’m not simply dismissing without reason the work of scholars who don’t believe in biblical authority.

The problem is that with Job, these scholars begin with a premise (that the text is not authoritative) that will allow them to go down any road they choose in interpretation.

I believe I am correct in saying that there is no textual evidence to support the view that Job did not originally contain the ending. The Masoretic text and the LXX both have the complete text of Job. That means that we do not have a text that gives us Job without the ending. Therefore any claim that Job didn’t have the ending must be speculation–speculation from thousands of years away, no less.

As for the difference in style between the narrative and the poetic portions, why is that such a surprise. Newspaper articles today have a “lead” paragraph to grab interest or introduce the material–followed by the content of the body, and then a conclusion. Each of these can be different styles of writing within the same piece. Just because the styles of writing in Job differ does not by any means necessitate the ending not being present.