I'm not sure whether people unfamiliar with "In Yo" get from reading the blog that this is an established aspect of Internal (Martial Arts) Training. Even in case some do, that is not to say all folks view In Yo the same, even within an Internal Martial Arts paradigm. Also since the quotation itself does little to explain how this 2+8/3+7 etc. model actually transfers on the physical (Internal Paradigm) plane.
So, the conclusion that Morihei Ueshiba was an internal martial artist based on his reference to this quotation, I fear, will for the uninitiated remain a gigantic puzzle. Or at least a big leap of faith.
I suspect the same goes for the next part of your concluding question.
Since your blog is addressed also/mostly(?) to those outside the Aiki/IP/IT/IS etc. paradigm, I'm not sure whether your first three excellent questions....
....automatically lead to the suggestion Ueshiba was an Internal Martial Artist. Even though you know I believe he was
To answer the question of connecting the link to internal power -from Ueshiba to China. It's a no brainer and irrefutable. It is in black and white in Ueshiba's correctly translated works with their counterparts being wirtten in translated Chinese works and in the history of budo in Japan.
It hasn't appeared anywhere because the authors did not have the skills necessary to the task. They could point and speculate but lacking the physical skills and not having the translation ability was a significant hurdle.
If you look to the literature we have discussed you will find innumerable references; both in specific reference and in detailed application. I am sure at this point you can see it action in the ICMA and in Aikido as well. An interesting study is to compare the movement of Tohei and Ueshiba it is clear that Tohei -while powerful-didn't get it in its fullness and was not incorporating the same method of movement as Ueshiba whihc more fully incorporated this model.
In and of itself it is interesting to understand how a link between; In/Yo ho, Heaven/earth/man and the above passage from the tora no maki is a requirement, the former empowers the later, otherwise people can only mimic with limited results. It's a pure comedy that a few Japanese shihan have shown it, but crippled as they are by their Japanese teaching model, it is all but useless to train with them and hope to ever get anywhere.