I haven't missunderstood the points - though I have, I fear, over-simplified the presentation for illustrative purposes. The key missing ingredient is that when I wrote that:

The baron must model the countess as seeing his decision as a fixed fact over which she has no influence,

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.

It is just that a Countess designed a particular way can be blackmailed.

Indeed. Those are conditions that allow the countess to be blackmailed.

Could the countess plausibly raise herself to a superior epistemic vantage over the baron, and get out from under his thumb? Alas no.

If the countess is already in an inferior epistemic vantage point, she can't raise herself deterministically to a higher one - for instance, she cannot stop treating the baron's actions as a fixed fact, as an entity capable of doing that is not genuinly treating them as fixed already.

The rest of that section was a rather poorly phrased way of saying that two entities cannot be in superior epistemic vantage over each other.