If we're in a "simulation", ie, in a computer program, there is a possibility we may find a bug, an exploit, or a backdoor, that would let us become "superuser"

If we're in a "simulation", ie, in a computer program, there is a possibility we may find a bug, an exploit, or a backdoor, that would let us become "superuser" and do magical god-like things (and most probably, just crash the process, destroying the universe and all there is inside it). On the other hand, simulation or not, this doesn't exclude or prove the existence of God. A simulation could be generated by unconscious physical processes in the super universe, as they could be created by "people" in that super universes (ie. gods). If the universe is not a simulation, it can still be a creation by a God, or happen to be by itself (or some unconscious meta-physical process). And as you note, it's also orthogonal to the question of whether the universe is a closed system or not. (both information-wise and energy-wise).