Nonexistence proofs are very difficult, so much so that it is generally considered reasonable to instead say:
There is no existing evidence or consistent interpretation of existing evidence that would lead us to believe this to exist.

Agreed - ish (see below).

This is typically the position of atheist.

Yep.

If there is existing evidence or consistent interpretation of existing evidence then this statement is no longer valid.

Not necessarily. "Goddidit" is a consistent interpretation of any evidence. Because it's consistent with anything, it does not invalidate the previous position.

To invalidate that position, there must be a) evidence and b) an interpretation that is not only consistent with the evidence, but inconsistent with potentially different evidence (the requirement of falsifiability). Now, your UO may well be potentially falsifiable. Well and good, but I still have difficulty with your evidence that it's purporting to explain. I don't think it's true that nothing happens unless observed - David Deutsch seems to think that everything happens, and we're only aware of a single path through the multitude of orthogonal universes thus produced.

So, until you can demonstrate that the "nothing happens" interpretation is the correct one, then your UO becomes merely a potential explanation of a possible interpretation of the evidence.