Would it not be more in keeping with the theological objectives of the Hebrew Scriptures (as a commentary on the relationship between humans and G-D) to focus first and foremost on the theological message of the Genesis account of the Great Flood? And in so doing, one would note that there are other stories of the Great Flood from ancient Sumeria and Babylon; and one would note that the Genesis account deliberately borrows the main story line from the Flood story that featured Utnapishtim, but also deliberately re-casts the story to reflect theological convictions of Ancient Israel.
See, for example, what Nahum Sarna says about the Genesis Flood story in his book, Understanding Genesis. In this approach, attention to the literary character of the story leads to a greater appreciation of its theological message.
So my question is: what can an insistence on the literal, historical and geological veracity of the Flood Story ADD?
As to the rest, I'm quite familiar with the argument. We could have a long friendly chat, but the bottom-line answer is fairly simple.
The reason why Christians "insist" on such things is that God's revelation is framed in history as well as words, and the words reveal the history (Hebrews 1:1, 2).
In other words, quite literally from Genesis to Revelation, God (A) DOES NOT reveal Himself by means of a series of high-flown apothegms and theoretical musings, but by starkest contrast (B) DOES elect to reveal Himself in words and deeds done in this year, in this kingdom, in this city, by this river, to this person with this name. There simply is no getting around it.
And Jesus affirms this very approach to both Old (retrospectively) and New (prospectively) Testaments.
So it all boils down to this: is Jesus a deluded liar, or isn't He?
A Christian is a person who affirms that He is the truth.
A person who denies that is a non-Christian.