simpleton: won't each side choose to cooperate, after correctly concluding that it will defect iff the other does?

Only if they believe that their decision somehow causes the other to make the same decision.

CarlJ: How about placing a bomb on two piles of substance S and giving the remote for the human pile to the clipmaximizer and the remote for its pile to the humans?

It's kind of standard in philosophy that you aren't allowed solutions like this. The reason is that Eliezer can restate his example to disallow this and force you to confront the real dilemma.

Vladimir: It's preferrable to choose (C,C) [...] if we assume that other player also bets on cooperation.

No, it's preferable to choose (D,C) if we assume that the other player bets on cooperation.

decide self.C; if other.D, decide self.D

We're assuming, I think, that you don't get to know what the other guy does until after you've both committed (otherwise it's not the proper Prisoner's Dilemma). So you can't use if-then reasoning.