One small point. According to relativity, space and time are not separate entities, instead, there is a 4-dimensional space-time continuum, so anything that exists, exists in all 4 dimensions (and it make no sense to say it exists in 4 dimensions simultaneously because time is one of the dimensions!).

Concerning the question of the provability of reality: Who or what is asking, or imagining it is asking, the question?

no provable reality exists except for one person at one time

I think you are begging the question. If you assume that other people exist, you are implicitly accepting the existence of an external reality.

I can assume other people exist but that does not prove they do.

First, can you even prove that you exist, or do you just assume that you exist? If you can prove it, please show the proof. I think you would find that any such proof would have to assume an external reality, and then you are caught in a circularity.