[D,C] will happen only if the other player assumes that the first player bets on cooperation

No, it won't happen in any case. If the paperclip maximizer assumes I'll cooperate, it'll defect. If it assumes I'll defect, it'll defect.

I debug my model of decision-making policies [...] by requiring the outcome to be stable even if I assume that we both know which policy is used by another player

I don't see that "stability" is relevant here: this is a one-off interaction.

Anyway, lets say you cooperate. What exactly is preventing the paperclip maximizer from defecting?