I think, from Tenryu's description, Ueshiba did aiki age on him. You say that the "secret" was that he was immoveable. I say that the secret is in aiki age, which saps the strength with which the opponent would move you. Making him weak is the same as making yourself strong. You like to appear to make others weak, but it doesn't really affect the truth.
Actually Ueshiba did quite a bit of having folks pushing him now didn't he?
Pushing his head, waist, chest and knees etc etc. As for immovability what you fail to get is why that is such a profound step, David. And AIki age is a huge mistake to bring up bud. Aiki age IS peng jin. And its tough to say "sap your energy" to me. Do you know "Why" its "saps your energy? Thats a bad terminology but I'll use it for the argument.
More fun is anti-aiki. Not playing the Japanese game and stopping or stalling the possibility of it being used on you. These are not strategies or tactics they are body conditioning and rewiring. Some of which you then don't have to think about in use , others you choose to use.
And immovability is the source for all the other things that are highly mobile and retain the essence of the immovability in -your own- heightened and faster ...mobility. The central pivot is patently useless without the essence of immovability. And all the later fun stuff still starts with that building block. There are means and methods to putting this stuff together. Like most things you need to get 1.. before 2... then 3... and so on.
Other than insulting Mike I don't get what you mean by unnatural though. This stuff -is- unnatural in every way. And the hardest thing -which most new guys who have met us will tell you -is that the mind gives out before the body.
See ya this fall