I'm pretty sure you're doing it wrong here.

"What if the structure of the universe says to do something horrible? What would you have wished for the external objective morality to be instead?" Horrible? Wish? That's certainly not according to objective morality, since we've just read the tablet. It's just according to our intuitions. I have an intuition that says "Pain is bad". If the stone tablet says "Pain in good", I'm not going to rebel against it, I'm going to call my intuition wrong, like "Killing is good", "I'm always right and others are wrong" and "If I believe hard enough, it will change reality". I'd try to follow that morality and ignore my intuition - because that's what "morality" *means*.

I can't just choose to write my own tablet according to my intuitions, because so could a psychopath.

Also, it doesn't look like you understand what Nietzsche's abyss is. No black makeup here.