The existence of existence, the existence of anything whatever, begs the question ‘how come?’ (what caused it?), and even the question ‘Why?’ (for what purpose?). If there was no existence, if nothing whatever existed, there would be nothing surprising in the fact. (Of course, for any question to arise, there has to be someone aware of existence and able to ask the question; if existence existed but there was no one to see that, there would be no question asked.)
The idea of God is an attempt to answer that question (indeed, both those questions). But this answer is logically inadequate in that it involves a claim to the pre-existence of God, i.e. to an existence which in turn calls forth the same question(s), bringing us back full circle. On the other hand, assuming the atheist alternative does not erase the question, the wonder at the existence of existence – so, it does not solve the problem either.
Once we posit God, we are forced to turn our attention to the nature of God, and discuss his unique Oneness despite presumed multiple attributes needed to explain His thoughts and actions. I think the answer to that is largely to found in the idea (expressed at the conclusion my essay The Five Skandhas Doctrine) that the spiritual domain is radically different from the material domain, without substance, space and time, or any other analogical features. This realization comes from the awareness that all our discussions of spirituality proceed by means of analogies from our material experience: the concepts we resort to are materially biased and therefore inevitably distort the ultimate reality somewhat.
The discussions of Classical Theism and Divine Simplicity that you mention are, of course, all relevant and interesting. And yes, Future Logic was my doctoral dissertation (feel free to write me by e-mail if you need some clarification while reading it).
Something about you (optional) logician-philosopher