As to your last point, yes, I think this all is very much what Tohei was getting at with "keep one point." But how? What does that mean? How do you work on it? Turns out there is a history and methodology around that.
I agree with Hugh here in several areas. Back in the mid 70's I took some ukemi from Tohei Sensei, went through the ki exercises presented at the time and can see that the 4 principles that he talked could be connected with the solo exercises from Dan that are being worked by many of us now could be aligned with the 4. The problem then was there was no explanation, at least with the folks who I can in contact with.
Every time I heard some one use the term "keep one point" I ask how do you teach that and what is the feeling, the manifestation of that? Never got an clear approach to answering this until I started going outside of Aikido, getting hints that allowed me to put together some of the bits and pieces I have gotten over the years within Aikido. Dan put the final pieces in place to help me in my efforts to figure it out (as far as I have).
Just some thoughts here (partial connections)..... keeping one point is in part pulling silk, extending ki is using intent to pull silk, relaxing completely is getting the shoulders out of the mix, weight underside is sinking or dropping (not always a manifestation that can be seen physically) shoulders under elbows under hands and a lot more.
This whole thing is like a giant puzzle that has so many pieces with everyone working to put it together. It seems that many having put enough pieces together to see something that is comfortable for them, fits on top of what they have already are good with that and stop searching. I am ok with that and with everyone doing what they want. As for me after nearly 40 years of training and heading toward 72 years of age......I don't see that we even have the outer edge of the puzzle together. I know that old friends like John Clodig and Walter Muryasz, along with a new friend Dan Harden have helped me see more of the whole than I got to earlier. Along with other I have met along the way, Mark, Marc, Sean from i Liq Chuan and many more that helped with adding pieces.
My point here...get out and see what is out there.....then play with it, ask for hands on help and see where the pieces fall.