Re: Tohei, Solo Practice, and Internal Training
Naturally, paired training is inherent: whether strictly regarding waza/kata, and/or to test the qualities of aiki/ki development.
In any case, history clearly shows that Ueshiba and Tohei's solo practice was heavily, perhaps primarily influenced by the spiritual practices you outlined, Graham. But given that aikido directly derives from Daito-ryu, that art's influence on aikido's solo and paired training methods cannot be discounted any more than the debt owed to the latter's technical syllabus.
The key point is that using the spirit/mind to lead ki in accordance with in and yo is common to multiple training methodologies expressly targeted at developing ability to use aiki in Daito-ryu and aikido, and the primary, common goal today is to improve one's self and have a positive influence on others through such practice. Tohei's time in Hawaii has had lasting, strong effects here. I'm comfortable with what I've said above, given that those folks who've I've trained with who've followed both the IP/IS methodologies as well as Tohei's agree they are symbiotic.
There's my 2 pence for the sake of promoting ai/harmony regarding this topic.