I am understanding that you believe that when the first human did what you explained above, he bypassed the possibility of there being just ONE god and immediately inferred that it was multiple gods ...all at once. You believe that there was no gradual rationalization from one god to many.
Who knows, that precise moment was not recorded. What is evident is that the earliest record does show animism/multiple spirits. I'm guessing that the spirit in the fire was distinguished from the spirit in the water for obvious reasons. Multiple spirits is far more intuitive than a single one, as I said, perhaps an even superior idea theologically.
Further, that this happened in a 'vacuum' where the idea or most minimal concept of a god was inexistent.
I don't understand why you say this happened in a "vacuum". The mind is a dynamic thing, the world is dynamic and filled with inspiration. I would, and have said elsewhere, nothing creative takes place in a vacuum. Humans are very good at copying and combining concepts. The concept of animate invisible spirits/gods is a projection of our own sense of agency/self upon an unexplainable action. We then, as I said, imagine the spirit to be like what we are familiar with, a human form or animal. No, creative/imaginative ideas are not from a 'vacuum', they are drawn from our own psychology and environment.
There is no "instinct" to believe in a god. The mind makes inferences (door closed so must be a closer) but how these inferences are interpreted has varied through history. There is nothing anymore instinctive about monotheism than there is about superstitions about black cats. Either particular tradition is the result of millennia of refinement and cultural transmission.