Thank you, Takahashi Sensei! That was a beautiful exposition on the commitment we make to both ourselves and to Aiki...and to anything in general. I believe that at the end of the day, regardless of the art we choose, we're engaging in an individualized study...it just happens to be along side other folks. I recently read an article which described doshu's view as being something along the lines of, "we're all doing Aikido, even though we're not all doing the same thing." The article went on to suggest that as long as we're open in our practice (interacting with each other), it doesn't matter if we're in different places with our training...and arguing over differences is rather pointless.
As a consequence, while I agree with the idea that aiki can refer to something very discrete ("internals"), it doesn't necessarily always mean that. Like you said, it's a word, and I think that means there will always be variance in comprehension and application. I think that while people who want to follow in the footsteps of kaiso ought consider internal training as a central pillar to understanding physical potency, I don't think it is necessary in order to call what they do "Aikido."
Which brings me to this idea:
..."more than the sum of its parts" assumes that the parts are all in place.
I think I disagree. Are there any students who ever have "every" part? Particularly if we consider the spirituality of O Sensei? We don't all have to have IP in order to enjoy the rest of Aikido. I think that was the point...though I could be wrong, of course. It happened once before...although in retrospect I may have been mistaken.