When people define a deity (God) as omnipotent, there are plenty of fallacies that can be drawn up, including the age old "can God make a rock so heavy that he can't pick it up?" question.
"Since the universe is everything that exists, God is a part of the universe."
One could easily do away with this statement by saying "God doesn't exist," and is thus not part of the universe. That's one solution.
"In order for God to have created the universe, His existence would have been mandatory prior to creation."
Most monotheists look at that statement, say "correct," and then claim that everything before the beginning of the universe (the big bang, whatever) is merely a "mystery" to us. When we say "before," it makes no sense if time was part of what was created. Perhaps there is an *eternal*, *outside-of-time* source for the creation (which one may identify as "God" in the sort of deist sense that physicists like Paul Davies acknowledges). I don't know what you think of higher-dimensional-theories, but what if the source for the creation of the universe is self-contained...like, after all the black holes suck up all the matter, ALL THAT MATTER is the source for the big bang...not a bigbang/bigcrunch cycle, but one self-contained cycle (like what Wilhelm proposed as "eternal recurrence" in Xenosaga III). If this were how the universe functioned, could "God" (whoever/whatever metaphysical intelligent being he/she is), be behind all of it? Maybe...if there is such a possibility as "disembodied intellect."
If I were to find "a" flaw, I would say the flaw is that you're working only with what you know, and we humans DON'T know a whole lot about the nature of the universe. we have some pretty fancy guesses (some of which are more plausible than others), but it's simply too difficult to make decent, logical statements about God and the nature of the universe when we *absolutely* know little to nothing about it.
Ramza
PS - I remember years ago when I was a more fervent, standard-Evangelical-Protestant type and I had an argument with you about God and Jesus and all that stuff. You Pwned me hard. I think I'm finally starting to grasp the sort of things you approached me with, and though I'm still Christian (in the "accepting the theology" sort of way), I promise not to be a douche bag like I was back then and tell you "accept Jesus and then it'll all make perfect sense!"