I guess the question is - what is the "optimal" method for Aikidoka who are interested in pursuing this avenue, and how to approach it. Particularly, if one's teacher (e.g. you mentioned Tohei) isn't being explicit on the intricacies and details. Or would one be better off pursuing other avenues, such as sanchin with Ushiro (again, for example)?
Well, I realize that people like Ushiro, Dan, and some others may see an "All Roads Lead to Rome" scenario, but I don't. Neither does Tohei and neither did Ueshiba... there are choices and some of those choices lead in somewhat different directions, even if all of those directions can have some aspects of legitimate "aiki" usage. If any way of training would do it, they wouldn't have specified certain tenets.
As I commented earlier in this thread, "Aiki" as a usage of intent and kokyu skill is not all that hard to do; the whole-body training is a bigger subject than just "aiki".
I'm not saying any particular approach is "wrong" per se. In my opinion some skills, no matter how developed, are better than no skills. However, if someone wants to replicate Ueshiba's skills in Aikido, I think some attention has to be paid to exactly what he did and how he trained. Otherwise you can wind up doing a "hard-style muscle-jin imitation of what Ueshiba was doing" while thinking that you're doing pure "Aiki"-do.
Tohei spent a lot of time up close trying to replicate Aikido as Ueshiba did it, so Tohei's intellectual and physical understanding of Ueshiba's approach to the ki-skills is probably more valid than most peoples'. The problems with Tohei's descriptions though are twofold, in my opinion: (1.) Tohei didn't fully have all of Ueshiba's skills (IMO) and (2.) Tohei's explication of how to train the skills wasn't very complete.
Still, Tohei's general exposition of how to attain Aikido Approved I.S. Skills (tm) had to receive general approbation from Ueshiba or Tohei couldn't have gotten the seal of approval for "This is Aikido".
Tohei's approach can be seen in that 5-part set of videos on YouTube and you can see that it looks nothing like Sanchin. Personally, I could probably use Sanchin as an example and still edge the topic/training around to Ueshiba-like skills, but that has little to do with the choreography of Sanchin... it would involve more of a "here's the basic principles and here's some of the variations" approach. BTW.... remember that "SanChin" translates as something along the lines of "Three Antagonisms": that refers almost certainly to the Three Contradictions or, in other words, Six Directions training.
Another thing to think of: Ueshiba did a lot of private, solo training. He used super-heavy garden implements, weapons, various forms of Aiki Taiso, and so on. Plus he used various breath techniques.
And a last thing to think of: It's unclear how much of the complete hara/dantien usage that Ueshiba had. My opinion of what he knew has grown over the last 4-5 years.
So, what's the optimal training method? I have a general idea that encompasses some "I think Ueshiba may have also had skill "X" that I have to consider, so I'd have to sit down and maybe write out a list. I haven't formulated such a list yet, but I guess I could do it. The problem is that to write it out on AikiWeb would mean that I'd get caught in a morass of explanations in order to explain what some terms, etc., mean to a readership most of whom have no background or common-baseline of terminology. Wouldn't this be easier to do on a forum like QiJin where the baseline is established in the archives and there are a number of people from Aikido who can chip in to the conversation?