Peter - Just grist for the mill - and perhaps anticipating part of your discussion: One thing that makes Ueshiba (and Takeda Sokaku) somewhat unusual is that a lot of ryu founders were also "lone-wolves" and subsumed their lives to an ascetic ideal of training. Yet either they, or one of their successors, each founded a ryu, with a formula (progression of teaching, "capped" by gokui, which, in theory, at least, enabled successors to replicate the founder's journey.
Just to toss in a more removed perspective: The basic documents and sayings of the many different styles and ryu's indicate some obvious commonalities and most of the commonalities are found not only in Japanese arts, but also in Chinese, "Indonesian" (some of which are more overtly Chinese-derived), Indian Buddhist/martial training, and so on.
The point I'm getting at is that no matter how you paint or cloak Ueshiba's art, it's still a subset of a classifiable genre. As was Takeda's and Tomiki's and Shioda's, and so on. There are other related arts that use the same basic principles and which have the same basic tenets, foundations, gokui, and so forth. I.e., the art of Aikido is not really all that unique in the sense that it is a singularity; therefore, its "transmission" should not be so convoluted that it requires laborious ruminations. If we could synopsize the transmission of one of the ju-jitsu-ryu, one to which we were not emotionally attached, then we should be able to analyze the transmission of Aikido similarly.
Interestingly, my impression of a lot of the Japanese teachers like Tohei, Abe, Sunadomari, etc., is that they also focus more on the bigger picture and worry less about the exact replication of O-Sensei's touch in their Aikido. If we're not careful, we can too easily become like those German clubs that exactly duplicate all the intricacies of every aspect of "real cowboys".... while the rest of the American West has moved on.