I'm looking at it and it looks coherent, though I'm in a bit of a fog so I may be looking at it through a rose-tinted lens. There's:

• the 'Standard Version', dating to around 1000 BCE or a bit earlier, in 12 tablets, composed by Sîn-liqe-unnini. This is the best known version. When people refer to 'the epic of Gilgamesh', this is usually it. Most fragments come from a library that got burned in 612 BCE (I'm guessing this is where your 600 BCE date comes from).

And a bunch of other fragmentary poems, mostly in shorter fragments survive (though the Schøyen tablet is a complete tablet). A few of these correspond roughly to known portions of the Standard Version; in other cases, their context within the Standard Version is unknown, or they may not have any real correspondence to the Standard Version. These include:

• Five known Sumerian poems, which are relatively intact, and which include the 'five original poems' that you name. These aren't 'the original poems' -- no 'original' version exists -- but they are certainly a lot older than the Standard Version. They don't have much correspondence to the Standard Version.
• A bunch of fragmentary Babylonian poems dating to the 18th century BCE. At least two of the fragments come from a single poem (the Pennsylvanian tablet and the Yale tablet). These are also a lot older than the Standard Version.
• A handful of late 2nd millennium fragments; some in Akkadian, some paraphrased into other languages like Hittite.

The Sumerian poems don't have much correspondence to the Standard Version. The Standard Version does show influence from some of the poems in the second category, however (but influence shouldn't be assumed to be direct). The third category is best regarded as derivative from the Old Babylonian versions, like the Standard Version, but independent of the Standard Version.

Anyway, the Gilgamesh poems floating around in the 2nd millennium were diverse. The Standard Version represents a condensation of a bunch of traditional stories into a single storyline, by a single poet. It became the best-disseminated version of any Gilgamesh poem, and survives in over 70 fragmentary copies, found in Nineveh, Babylon, Ashur, Sultantepe, and Nimrud. The latest fragment dates to around 130 BCE.

The older fragments are of intrinsic interest, of course, but are also of interest because the Standard Version is fragmentary. People like to have supplementary information to fill in gaps. There are a handful of places where older texts can 'fill gaps' in the Standard Version, but it's wise to be cautious with that kind of supplement.

Of the older fragments, only one has any overlap with tablet 9 of the Standard Version, namely the Sippar tablet (London BM 96974 + Berlin VAT 4105), dating to the 18th/17th cent. BCE. Andrew George's Penguin edition of Gilgamesh uses this text to insert 14 lines into his translation of the Standard Version: he places it between lines 18 and 38 of tablet 9. So there's some basis for seeing tablet 9 as based on a much older model, at least in part. The surviving text of those lines in the standard version is very fragmentary.

Rather, we don't have an Old Babylonian version to compare it to. So Old Babylonian counterpart to that part of the poem survives.

I‘m not quite sure what you mean by that, so could perhaps elaborate? The old Babylonian version of the rivers etc. has not survived, yet the existence in the standard Babylonian version etc proves that it most likely existed? Or did I misunderstand you?