I think you've all seen enough PDs that I can leave the numbers as an exercise

Actually, since this is an unusual setup, I think it's worth spelling out:

To the atheist, Omega gives two choices, and forces him to choose between D and C:

D. Omega saves 1 billion people if the Earth is old.
C. Omega saves 2 billion people if the Earth is young.

To the creationist, Omega gives two choices, and forces him to choose between D and C:

D. Omega saves an extra 1 billion people if the Earth is young.
C. Omega saves an extra 2 billion people if the Earth is old.

And then -- I hope -- you would cooperate.

No, I certainly wouldn't. I would however lie to the creationist and suggest that we both cooperate. I'd then defect, which, regardless of what he does, is still the best move. If I choose C then my action saves no lives at all, since the Earth isn't young.

My position on one-shot PDs remains that cooperation is only worthwhile in odd situations where the players' actions are linked somehow, such that my cooperating makes it more likely that he will cooperate; e.g. if we're artificial agents running the same algorithm.