It's established that solo training to develop aiki has been done in Daito-ryu (by Sagawa and others) and aikido (by Ueshiba [who was an exemplary Daito-ryu practitioner] and Tohei in particular). So, isn't it reasonable to say that if Tohei was devoted to aikido, and Ueshiba (if not his son so much) seemed to acknowledge the approach and value of what Tohei was doing, then Tohei was combining the Daito-ryu-based approach he learned from Ueshiba with shin shin toitsu methods? Did Tohei ever expressly disavow the influence of Ueshiba's teachings regarding solo methods for developing the use of aiki/ki?
I wouldn't say so. In fact I would say quite the opposite. Without others you cannot develop.
Once again I think it is just something given too much significance and no perspective. This alone leads to such statements as you give above.
There is solo and there is joint or with others. They are interdependent. In fact in anything you do you will find there is sequence, one follows the other.
Every serious practitioner and indeed every athlete or anyone practices solo as well as with others.
As Aikido is an art in which action is 'with others' ie: opponents, then I think that puts the balance back into such statements.
As I said, Tihei practiced shin shin toitsu, a solo practice. Ueshiba studied Omoto, a solo practice. All great past masters studied some such be it zazen or whatever. Many solo practices and that's not including when you get stuck and go away trying to work it out....yet another solo practice.