Re: Principles of Aiki
I have been following the contributions of Mike Sigman, Dan Harden and yourself on these topics from a distance but with great interest.
A few years ago I discovered that the shoulder blades were somehow crucial from a completely independent source: Scaravelli Yoga. In this way of practice (as I have learned it, at least), the arms and legs are connected very directly with the spine - not rigidly, but in a very alive way. This is quite different from the emphasis on outer form and superficial stretching that you might find in many yoga classes. As far as the shoulders go, they are not dropped (as you might instinctively try to do when told to "relax"), or "stretched" outwards, but suspended as if each shoulder blade were sitting in its own hammock, and with a little tension in towards the spine.
The two most strikingly beneficial effects of my yoga practice on my aikido are an increased awareness of my feet, and a growing sense that inner power comes from the spine, rather than primarily from the belly as we were taught in the old days. I find that when I show my students how to connect the arms through the shoulder blades to the spine, everything suddenly works (well, sometimes...) with much more power and less effort.
What you say about the tailbone is also fascinating, and again close to what is taught in the Scaravelli yoga tradition. We used to be encouraged in aikido to push the tanden forward and to tilt the front of the pelvis downward: this resulted in excessive curvature of the lumbar region, and the tailbone to be tilted up and back (and incidentally often to long-term spinal damage). My present yoga teachers, by contrast, teach that there should be a "fullness" in the lower back, and although they tend not to talk very explicitly about the tailbone or pelvis, results in the coccyx being aligned more forward, under the pelvis. Again, I have found that this has really improved my ability to relax and deliver power in aikido. The hard part of this, in my experience, is that to get this alignment of the spine to happen naturally, the front muscles of the thigh (quadriceps) need to be quite soft, and mine certainly aren't, although I'm working on this!
Interestingly, when I watch my own teacher (Kanetsuka Sensei), I notice that his lower spine is almost straight, as have many practitioners of Chinese arts, while many other senior aikido teachers have that pronounced concavity in the lower back which pushes the tailbone backwards.