"I don't really see why we need only one first cause rather than one first cause for each causal series (i.e. the will of the person moving the rock is the first cause there, the earth is the first cause of the lamp-chain-house-earth causal series, etc.)" Think about this way, the will of the person directs itself towards something. In other words, the actualization of a potential. Since, one has potencies then they cannot be Actus Purus.

Yes, also another issue: What is it that keeps all those massive bundle of attributes "together" and prevents them from "separating"? 

Lately, I've been thinking of Divine Simplicity. If God is not simple then isn't He a massive bundle of attributes (all-knowing, all-good, all-powerful, etc.)? Quite a bizarre view of theism.

@ Jeremy Taylor
I forgot to mention Michael Hanby's No God, No Science: Theology, Cosmology, Biology. I heard his book is pretty good and has a Classical Theist touch to it. Also, philosopher William E Carroll has done some work in contemporary science and classical metaphysics.
 

Well, at least Schellenberg and other atheist philosophers present more thoughtful and systematic arguments.