Re: The Descent of Aiki
If we go by Sokaku Takeda's own thoughts, we find that in his eimeiroku and shareiroku are listed two of his teachers: Shibuya Toma of Ono-ha Itto-ryu and Chikanori Hoshina. (Stan Pranin Converstaions with Daito Masters.)
I have reason to believe that in some of the kenjutsu schools, there are/were training methods which rebuilt the body such that it created "internal structure". It's possible that some of these schools might have had a bit more training in internal skills than just that. We can guess that Takeda must have learned some internal training from Ono-ha Itto-ryu.
Chikanori Hoshina (Tanomo Saigo)
Now, someone I know has a theory about what oshikiuchi meant. It has been translated as "inside the threshold". Ellis states that the Hoshina turn out to be Chinese. When a Chinese martial arts master takes in a student to train in the "secrets", this student becomes an "indoor student". The theory is that oshikiuchi was the Japanese way of stating what the Chinese did. In other words "inside the threshold" = "indoor student". Hoshina could actually have known Chinese internal skills *outside* of any martial abilities. (1)
Factor in the sumo training, his stating the two teachers above, and then the change in Japan where the sword was no longer carried. Takeda then makes the move to unarmed. His skill by now was very good and so when he meets people, they start interacting in very strange ways because of aiki. We have the beginnings of Daito ryu, a martial art created by Takeda but owing to his teachers for giving him the sound basis of internal training.
Takeda's eimeiroku and shareiroku officially state that he is Daito ryu Hombu-cho or Somucho. In other words, he is the director of a training method that he put together from a few different places and he gives two of them their proper due in his books.
(1) For example, spirals. The ability to have internal spirals is not dependent on a martial system. One can train them without any martial ability. Takeda could have learned internal structure and power from kenjutsu and sumo, but not have learned spirals. He could have learned spirals and internal power from Hoshina, who did not train in the martial arts but kept the family "secret" alive for "health" reasons.