This line of inquiry can’t go anywhere because the answer Monotheist will give to every question is that an objective answer does exist, it’s ordained by god himself, and conveniently lines up with whatever his feelings on the matter are anyway.

You know what’s interesting, though. Monotheist will talk about how we, people, are fallible and sometimes get morality wrong, without acknowledging that this fallibility forces us into a relativistic morality regardless of whether or not an objective one actually exists. And that’s because you’re supposed to read The Bible. Famously impervious to reinterpretation The Bible.