Just to quickly clarify. I don't believe That applying force where your target is weak, is ju. But rather, using the quality of ju allows me to yield to your force, which will protect me, and allow me to find a weak spot on you (if that is my goal).
I think that your sceptical approach is basically correct, although I don't personally think that someone can take an already-known term and apply a consensus agreement among a limited faction that uses the same term to mean a different thing. It becomes like the hookah-smoking caterpillar telling Alice that words mean what he wants them to mean. If that's the case then nothing can ever be defined.
In terms of Asian martial arts, there is obviously a consensus of some sort or Ueshiba would not have been using (in his writings and douka) the traditional terms about ki and ki's physical functions that go back hundreds, if not thousands, of years. The terms and sayings that Ueshiba quoted are the same terms and sayings found in a great number of Asian martial arts... wouldn't it be reasonable to assume that he (Ueshiba) was talking about the same "internal" things as the old-sayings he was quoting? In other words, Ueshiba wasn't making up any new definitions, he was abiding by old ones.
Physically/functionally, I haven't seen Ueshiba demonstrate "ki" things in any way that was outside of the known ki/qi demo's of China. When Ueshiba, Tohei, Shioda, and others, show these things in stark or at least reasonable isolation (outside of techniques), nothing they do is different from the known qi/ki demonstrations in a great number of arts.
What I'm basically saying is that there seems to be little demonstration of individual definitions of internal strength (whether it's called "aiki", "kokyu", "jin", "ki", "qi", or whatever), so a call for individual definitions seems out of place to me. If you posit that there is a unique definition for "aiki" in Aikido, then you're left with explaining why Ueshiba used so many references to the traditional terminology.