Do you mean that I am mistaken about the Japanese custom regarding seating and shomen?
Stefan, my apologies for coming back to this so late. I'd completely forgotten about this thread, and only found it again recently searching for something.
My disagreement is not with your understanding of the custom. Rather it is with the application of the custom. The rules of etiquette are to forestall insult, problems, and issues; ideally they should not cause
such problems. In my experience, Japanese people are perfectly willing and capable to bend customs and rules of etiquette if doing so promotes the greater wa.
In particular, when issues of translation arise, translators tend to be "attached" to the status of the person they are translating for. It's understood that their particular seating is set-up for ease to do their job, regardless of their actual status. Not that it's consciously thought of this way, but as an example, I often translate for the soke of Yagyu Shinkage-ryu. In these situations, as his assistant I'm temporarily elevated to his status, given that I am speaking for him. Despite the fact that the people I am translating to may outrank me or be my senior by many years, I sit at a position of much higher status.
I never met Nishio-sensei, but I have no doubt in my mind that he would have been perfectly happy to sit in the middle, with the translator next to him (closest to the shomen), having had it explained to him that you were putting him in the middle as the seat of honor in accordance to Western etiquette. It may have been a bit unorthodox (since you imposed Western etiquette upon the Japanese etiquette), but by no means absurd. Or, alternatively, he could sit closest to the shomen, the translator in the middle, and you at the other end; in such a case the translator would not be sitting in a seat of honor, but attached to Nishio-sensei's seat of honor (set up in the Japanese style). It would be less a matter of three positions, but three chairs making up two positions. Particularly if the translator was positioned slightly behind Nishio-sensei.
I wasn't there, and have never met Nishio-sensei, so it is admittedly presumptuous for me to suggest this, but it seems to me that Nishio-sensei's decision was less the resolution of a crux of etiquette, but rather a resolution of your personal problem. I think he perceived that the issues of etiquette were very important to you, and he responded in the easiest way to resolve the issue for you, without contradicting you or negating your feelings. If you had an issue with three chairs, then removing one of the chairs would solve the problem. I only say this because I've heard nothing but good things about Nishio-sensei's bearing and personality, so I find it hard to believe that he would be such a stickler for Japanese etiquette that he'd make the fellow translating sit on the floor rather than sit himself in anything other than the unequivocal Japanese kamiza position. And it seems to me that, while Nishio-sensei solved the issue in a way that saved face for everyone (except perhaps the poor translator), the whole issue could have been resolved right from the beginning, without even bothering Nishio-sensei, if you, knowing both Japanese and Western etiquette, had just chosen one to follow and gone with that. Nishio-sensei's response was good, but IMO this would have been ideal.
This is really a nitpick more than anything, and by no means should be taken as disagreement with the larger point of your excellent article.