Your proof falls apart on the first line.
In a multiverse all events and things will exist somewhere. That is the logic behind multiverse adherents dismissing the Anthropic Principle. The problem is that a Supreme Being is included in that set of events. Thus by multiverse logic the Supreme Being exists.
No, given the "finely-tuned nature" of the universe, we would seem to really only have two choices: either the universe was purposefully created (which would have to have been done by an immensely powerful intelligent being -- "God" by about any definition), or, if it happened by chance, then there must necessarily exist an essentially infinite number of universes in order to make it possible. The proposition that ours is the only universe, and that it is here by chance, is many, many orders of magnitude too improbable to believe.
Therefore, apparently, if there's no God, then we have to have an essentially infinite number of universes.