[Serious] Could an omnipotent and omniscient god prove that he was God?

[Objectivist]

It's a simple issue to resolve.  Grand is speaking of the observable universe, known universe, or near universe.  The difference will matter because imprecision or difference here will manifest exponentially all along the line.  If we correctly understand what the term universe means, and how Objectivist is employing it, then there can be no "limited range of universes".  The universe cannot, as all that exists, include things that do not exist.  Similarly, we cannot add anything to everything.  There's no "everything plus one" that's just another entity in "everything".

We're talking about the granddaddy of all concepts here in a discussion about a philosophical issue.  It's perfectly OK to have different definitions in different contexts because the reference of a concept is wider than the definition.  So if science studies matter, energy, space, and time then they can narrow the definition somewhat because their field of study is narrower.  Physcologists study the mind and human behavior.  Biologists study all life.  These things are all included in the philosophical definition that I gave.  If we come across some new form of matter or thing that we didn't know existed before it is automatically included in the concept 'universe'.

This is where a theory of concepts is crucial because it includes as the final step in concept formation the rules of definition which are not arbitrary but based on the mental process of forming concepts and their use in cognition.  They are based on how our minds work which makes them objective rules or principles.