1. You presuppose that if God acts on a prayer, it indicates that he was previously unaware of something.
Umm, no, he didn't. He simply said if God is unaware of something until someone prays, he is not all knowing. If God is ever unaware of anything, he is not omniscient. That seems pretty basic. To say otherwise is to render the words all-knowing and omniscient meaningless.
2. You presuppose that if God is all-knowing and good that he must necessarily enforce what is best for people.
Where did he imply any such thing? If God is all-good, he must only do good, whether or not anyone asks him to. How such actions relate to people isn't necessarily relevant to whether or not they are good.
3. You presuppose that, as a finite, relatively insignificant human being, you could possibly know whether and when God intercedes in our world and to what ends.
No, he just used basic logic. If God is not bound by that, he is incomprehensible in a way that would leave us unable to know anything at all about him, so any argument about God would be as silly as an argument about whether or not 1 is the same as 2. Also, if God is not bound by logic, I'm curious how you resolve the question of whether or not God can create a mountain so heavy that he cannot move it. The traditional resolution of that paradox is to say that such a thing is logically impossible, and while omnipotence would allow God to do anything that is possible, it would not allow him to do the impossible. Do you have a resolution every other philosopher and theologian has missed?
4. You presuppose that you could even know what is "good" or "best" from the perspective of an all-knowing, all-powerful, universe-creating, life-breathing entity beyond our comprehension.
Once again, he neither said nor implied any such thing.
The GP poster never claimed his argument disproved the existence of God in general, just a God with very specific characteristics. He didn't even imply that prayer was useless -- he explicitly allowed for the possibility of it having psychological effects on the person praying, a la meditation.
His argument showed one thing, and one thing only: That if God is all-knowing and all-good then his will and his actions cannot be influenced by prayer. That doesn't mean God is nonexistent, and it doesn't mean prayer can't have effects on the person praying, it just means that if someone can influence God's actions via prayer,
either God is not all-knowing, or he is not all-good. His point was this: Since religions such as Christianity teach that God is, in fact, both all-knowing and all-good, people who believe that such a God both has those characteristics and can be influenced by prayer are misunderstanding their own religion.
Except for atheists and agnostics
Actually, I've found that many atheists and agnostics credit the Bible for being the main reason for their (lack of) belief.
Without life, Biology itself would be impossible.