Your article is one of the explanations that I want to illustrate regarding "center" or "one point" ( 一点 ).
The image that shows the three planes intersecting each other at one point, was similar to what I have imagined. The only difference was that I imagined it in circular fields creating a sphere.
I agree with the egocentricity that you mentioned. There is a third center, like you point out in the article. The way I visualize it is that the uke and tori's center expand in a sphere and then it will meet in a tangent point creating the third center.
Of course this does not necessarily exclusive to seika-tanden/itten, it could also refer to the point when two opposing forces meet. For example: when nage is accepting a shomen, there is a point made between the uke's arm and the nage's arm, where the forces regardless which one is stronger, cancels each other out - the third center.
Well... I guess what I'm talking right now is gibberish because I'm trying to put the visuals inside my head into words. I guess it's not easy as it seems. Tried it in my multiply page