I'm sorry, I've just skimmed over all of this, so please let me know if I'm missing something...
Why would ANY atheist be a determinist? Wouldn't that imply a belief in some higher power (a "determiner," if you will)? I suppose it's fair to say that there are Judeo-Christians who believe in free will... but certainly this doesn't mean that atheists cannot support it as well. In fact, I think most of them do... I was explaining existentialism to an atheist friend of mine, and his response was "Oh, of course. What else would we believe?"

Well, I'm an existentialist, which means that I reject a higher authority, and believe in an individual's free will (and the responsibility that comes with having it). I think that, no matter where you are in life, you are free to make your own choices, and you alone are responsible for dealing with the consequences. It grinds my gears when I hear people say, "Oh well, it's God's will" or "I prayed about it, so that's all I can do." I don't believe in making excuses. You can always be better than you are.
I also don't believe in an absolute moral code. The Ten Commandments say that under no circumstances should one tell a lie. But if it's the Holocaust, and I'm harboring Jews, and Nazis knock on my door, I'm sure as hell going to lie to them, to save the lives of innocent people. An existentialist appreciates the privilege and the burden of making difficult choices.
As for the "meaning of life"... an existentialist believes that there is no one answer - it is what you make it.
If life is predetermined, what's the point? /:

Sweet Disposition, by The Temper Trap.
(: