(Come to think of it, blackmail threats of mutual destruction unless paid off, would seem to become more probable, not less, as you became more able to exhibit and prove your source code to the other player.)

I think Nesov's position is that such threats don't work against updateless agents, but I'm not sure about that yet. ETA: See previous discussion of this topic.

I take statements like "Obviously, everyone plays D at the end" to be evidence of very high a priori correlation - it's no good talking about different heuristics, intuitions, priors, utility functions, etcetera, if you don't actually conclude that maybe some players play C and others play D.

That doesn't make sense... Suppose nobody smokes, and nobody gets cancer. Does that mean smoking and cancer are correlated? In order to have correlation, you need to have both (C,C) and (D,D) outcomes. If all you have are (D,D) outcomes, there is no correlation.

How would that happen?

I'm referring to rock-paper-scissors and this example. Or were you asking something else?