If you assert something such as the existence of a demiurge, it's up to you to provide support for the theory. That the idea pre-exists modern science and is thus widely believed by default doesn't strengthen it in the slightest, from a philosophical perspective.

One has to ask why the notion of a creator is necessary to begin with; sure, we're lucky the universe exists as it does but on the other hand if we didn't need to breathe space travel would be a lot cheaper, so our current form is a limitation as much as an adaptation. And if this being does exist, prior to the universe as we know it, then where? In a higher dimension of some sort? In this context, the notion of a creator is a proxy for our lack of knowledge rather than a positive demonstrable.

As for multiple universes, try the number line. Consider how some functions can iterate endlessly upon themselves without falling into an oscillating or steady state. In abstract terms, what you have there is a one-dimensional universe populated by point beings whose only property is that of position, but which can yield exhaustively complex behavior nonetheless. All iterative functions 'exist' simultaneously for all permutations of their starting values whether or not anyone is actually calculating them at any given moment. You can expand this concept to include more than one dimension, or alternative geometries, or indeed any axiomatic formal system, with the one fundamental constraint being that completeness and consistency are mutually exclusive. We could do worse than consider our own existence from a similar perspective.