[Eric B]

Well, again, if you look at 9 as the pure Phlegmatic and thus completely ambiverted (both in expression and responsiveness or people/task orientation), then it is in the center of, and thus close to all of the temperaments. Still, as you noted, you would expect types like 1 or 3 to be next to 8 first.
But this would be to match the E/R scale, but as we know, the Enneagram theorists were not using E/R. The fact that you and others noted this about the 1 and 8 being next to 9 shows that E/R is the most "natural" matrix to map the types on. (And those who do compare them to E/R behavior generally see the 9 as low E/high R, which would make it diametrically opposite of 8. But again, I would say it was dead center).