implicit in that was the assumption that the baron was rational, knew his source and the countess' and would arrive at a decision in finite time - hence he must be correct in his assumption. I nearly wrote it that way, but thought this layout would be more intuitive.

You say here that the baron is rational and he knows the countess's. This being the case the only way for the countess to be blackmailed is if she implements a defective decision algorithm. Yet you describe the difference between the two as an 'inferior epistemic vantage point'. This does not seem like the right label. It seems to me that the advantage is instrumental and not epistemic.