I always saw the codex as more of a blueprint for the universe rather than the universe made real. I thought that it doesn't really maintain the universe because the universe was basically gut-punched out of the codex by Amul and Fate. The universe just is now. It's existence isn't really dependent on the continued existence of other entities.
Edit: That doesn't mean that the codex couldn't provide insights into the universe that are otherwise ambiguous to gods trying to work them out. It'd be really valuable in that sense. That's kinda not going to happen any more unless the godkiller reverts back for whatever reason.
Couldn't have said it better.
With regards to the Godkiller itself, there is nothing to say that it does not still containt all that information within it. From my understanding, it is currently in the form of a cup. But perhaps in the hands of another mortal it could take on a form which can unveil the information of the Codex.
But, as has been canonised in Vulamera's experience, looking into the Codex is bad in general. If it drove a god crazy, what will it do to mortals...
Unless, of course, mortals have some mysterious immunity...
So many questions, so little time