you can be reasonably sure that if both calls return the same result you are being simulated.

Shouldn't it be, at most: you can be reasonably sure that you are being simulated either a) after both calls return C or b) after you formally choose D having already seen that both calls return D?

If a simulation chooses C after seeing both results D, then the simulator might as well actually defect, so it does, and the non-simulation chooses C, just like the simulation.

If an agent strongly prefers not to be a simulation and believes in TDT and is vulnerable to blackmail, they can be coerced into cooperating and sacrificing themselves in similar cases. Unless I'm wrong, of course.

One really ought to make anti-blackmail resolutions.