If the place doesn't exist then it's a different model and there is no real structure (everything is relative to something else). And by structure, it is a hierarchical structure. like perspective A encompasses perspective B and C, so A is more 'complete' than those two. But perspective A + X is encapsulated by a larger perspective, which is encapsulated by a larger one, etc to logical conclusion. So it forms a pyramid. A pyramid has a top spot.
I wouldn't call it a fiction. More like something theoretical. But useful, yes.
It is not the highest vantage point relative to me, it is the highest vantage point period. Relative to everything. So, it would encapsulate all of reality, essentially.
Do I believe atheism entails post-modernism? No although I haven't put much thought into it. Post-modernism doesn't take into account an objective reality it seems to me, and I believe that that has to be taken into account. So not to do with atheism which I think has no inherent qualms with objective reality.
Do I think it is right to believe in things you don't hold to be true? I'm not sure what you mean. You can run under the assumption something is true without knowing it is true. I don't see anything wrong with that, if that is what you mean. If you have reason to believe that something is false and you still believe it then I see a problem with that, if that is even possible.
disclaimer: This isn't a topic I have thought about much so we're shootin the shit here