[Cushman]

But can we not conjure an omniscient observer as our frame of reference?

Yes. We can not. In fact, that is precisely what the theory of relativity states: There is no privileged reference frame.

There is literally, actually no such thing as "what exists in the space of the water vapor cloud as Earth time is still July 24, 2011". As I look at my clock at noon on July 24, I see water. Someone at the reservoir will not see my clock strike noon for twelve billion years, my time, and they'll look around and maybe see no water. We disagree, but we're both right in our own frame of reference. Both perspectives are true.

Which makes sense if you think about it; your omniscient observer would have to agree with one or the other (or some other) disagreeing perspective. What criteria could you possibly use to choose between them?