“That two states of comparable moral standing and incomparable standing in most other respects still do not recognize an otherwise near universally signed and respected convention, does NOT allow to conclude, as Keitner does in a remarkable short circuit, that diplomatic immunity under this caput would not exist.”
That the United States is not a signatory to a treaty allows us to conclude that the treaty does not apply in American courts. There is no short circuit here. The purpose of American courts is to enforce American laws within our borders–not to enforce international law contrary to the will of Congress.
That the US and North Korea, as nation-states, are of comparable moral standing, is a claim you are invited to defend. Start with pictures of the US prisons where the families of political activists are tortured to make them confess. Unless you can do that, the attempt to sketch all “rogue states”–apparently this means states which agree to different treaties than you would like them to–with the same cartoonishly oversimplified caricature fails.
The head of the IMF is no ambassador, which is the whole point here. So your colorful example in which Darker Ruritania kills an ambassador for les mageste has no relevance. Even if that were to happen, though, the proper response would be outrage over the barbaric penalty for a victimless crime, not outrage over a breach of international law. You have the cart before the horse. International law is an instrument for promoting justice, not the other way around.
Having the cart before the horse can make it hard to tell which way you are going. It seems likely that DSK sexually assaulted a young woman in New York. If you really believe that international law protects him from prosecution for this, solely because he is the head of an agency, shouldn’t you be trying to change international law? Isn’t it more important that international law protect the weak from abuse than that it be adopted in full by every nation? Yet most of its boosters seem to have more urgency about the latter goal.