"As someone who rejects defection as the inevitable rational solution to both the one-shot PD and the iterated PD, I'm interested in the inconsistency of those who accept defection as the rational equilibrium in the one-shot PD, but find excuses to reject it in the finitely iterated known-horizon PD."

... And I'm interested in your justification for potentially not defecting in the one-shot PD.

I see no contradiction in defecting in the one-shot but not iterated. As has been mentioned, as the number of iterations increases the risk to reward ratio of probing goes to zero. On the other hand the probability of the potential for mutual cooperation is necessarily nonzero. Hence, as the number of iterations increase it must become rational at some point to probe.