> Saying "there is no God" instead of "I don't know whether or not God exists" presumes a certain understanding of cause and effect
No, it may just be grounded in an epistemological approach in which entities which are not necessary to explain observations are presumed not to exist.
> To put it another way, one cannot be an atheist without a religion - one can only be agnostic.
Many self-described atheists reject the atheist/agnostic distinction as meaningless, viewing the absence of belief as equivalent to unbelief in their epistemology.
> To put it a third way, Cicero says "religion consists in offering service and ceremonial rites to a superior nature that men call divine." The atheist says "there is none to serve" - but that is patently untrue, for in that case there would be no end to strive after and all atheists would perish by inaction.
That is fantastic illogic. The absence of a "superior nature" to which one might have a reason to offer "service and ceremonial rites" does not deny that there are goals that one might strive for. Maximizing one's own experienced utility, for instance, requires no assumption of a "superior nature", nor even a rejection of solipsism, but still provides an end to strive after and motivate action.
> Therefore, all atheists have chosen to serve some superior nature, either themselves
Serving yourself is not serving a "superior nature", since "superior" is meaningful only in comparison to something else, and in Cicero's statement it is clearly in comparison to the self.