I think your ideas are wrong. I would be happy to continue the conversation from there if you are interested.
I have doubts about my ideas, too. However, if we take Takeda at his word, then he learned from two people according to his eimeiroku and shareiroku: Shibuya Toma of Ono-ha Itto-ryu and Chikanori Hoshina.
Looking at either Ono-ha Itto-ryu, Shibuya Toma, or both, could there be a training methodology that instills some portion of internal structure or skills?
I think certain ryu did have training that sort of rewired the body for internal structure. I also think some people throughout history in koryu had internal training and/or skills.
We are talking internal, so this would be outside any external technique or kata, per se. More like what one trains while performing the kata. I gave the example of the hips and cutting while moving forward. By training certain kata, that might distill in a person some internal structure over time. Did the Ono-ha Itto-ryu that Takeda studied have this kind of training?
In other instances, we need to remember Ueshiba and Sagawa's words. Aiki makes everything better. Aiki works in all martial arts. So, someone who had internal training could pass on that knowledge in any martial art. Did Shibuya Toma have this kind of training?
The theory is that if we take Takeda's word for whom his teachers are, we must look to Shibuya Toma of Ono-ha Itto-ryu and Chikanori Hoshina. What did they teach and in what kind of vehicle? Was Ono-ha Itto-ryu used?