Care then to make any comments on how well my paraphrased points align with OSensei's assertions? I'm not trying to be snarky, just reaching for clarity.
Very nice presentation of what I think is a complex question. I had the good fortune to meet, talk with, and take ukemi from the Nidai Doshu. I have read everything in English he either wrote himself or someone wrote about him but it is not a vast body of work that's available so it's not saying much.
In my opinion, the reason that Aikido exists world wide, with an estimated 1 million people training, is due entirely to Kisshomaru. His father may have created the art, but what he created was largely not well understood by his own students and was too arcane to spread widely amongst modern Japanese, much less overseas.
If you want to see what Aikido would have been without Kisshomaru, just look at Inoue Sensei and his art Shinei Taido (formerly Aiki Budo). It is generally agreed that Inoue (O-Sensei's nephew and main student in the early years) looked more like O-Sensei than any of his other students. Further, he was a life long follower of the Omotokyo Faith and therefore was aligned much more on a spiritual level with O-Sensei's own beliefs.
So you have a guy who was, arguably, the closest thing to Morihei Ueshiba you could find, and yet his art is practiced in Japan by only a small number of folks and world wide, if there are any, I haven't heard about them. I certainly do not know of any in the States. Now, that doesn't mean there aren't any but it shows, I think, what Aikido would have been like if the art had simply been presided over by O-Sensei throughout the post war years.
At a recent Rocky Mountain Summer Camp I sat with Stan Pranin and Saotome Sensei talking about post war Aikido. I asked them who they thought had tried the hardest to understand Aikido the way O-Sensei understood it. They agreed that Sunadomari, Hikitsuchi, and Abe Senseis were the three. I think it is revealing that, although I hadn't actually specified as such, both Saotome Sensei and Stan Pranin pretty much assumed I meant from a "spiritual" standpoint. The fact that no one even thought to make a distinction between the spiritual / philosophical side and the martial / technical side is a direct result of the fact that what distinguished O-Sensei in the post war years, especially, was his almost total concern with matters spiritual.
So Kisshomaru, the Nidai Doshu, is in the front line position to preside over the growth of Aikido in the post war years. Daito Ryu was barely known, even in Japan. It's later growth was actually sparked by the enormous popularity of Aikido (they sort of slipped streamed along behind Aikido) and the efforts made by Stan Pranin to familiarize both the Japanese themselves and the foreign public about Aikido's parent art.
So here's the Nidai Doshu presiding over an art which has no popular and well known antecedent, whose Founder was a Shinto mystic. Various folks maintain that he didn't understand his father, either from a martial technique standpoint, or from a spiritual standpoint. I absolutely disagree with that assumption. I think he understood quite well.
But he was tasked with taking the art to the world. He had to decide what aspects of O-Sensei's art would be beneficial to modern, post war folks. He decided, correctly I think, that the Shinto, overtly religious aspect of O-Sensei's Aikido, would not be understood at all by the typical modern Japanese citizen nor would it "travel well" outside Japan to other cultures.
I believe that O-Sensei in the post war years had already de-emphasized the martial aspect of the art in favor of developing a movement style designed to foster an understanding of various energetic and spiritual principles. The Budo aspect was important to him but not as "fighting" external enemies, but as a battle with the baser aspects of our own selves. So I think that one should look at Aikido as O-Sensei envisioned it, as a body art designed to reprogram the practitioner both physically and mentally to understand , on some level, the essential unity of the universe and what ones place in the whole should be.
I think that Kisshomaru simply continued that idea and refined what the goals of the training would be. He opted for a lass mystical, more ethical, behavioral model. Not out of keeping with the Founder's Aikido at all, but certainly less complex. It was an Aikido designed to make people's lives better. It was an art designed to create "gentlemen" in the most positive sense of the word (and, oh yes, women could do it too). Kisshomaru manifested that aspect in his own life to the utmost. He was a real gentleman, a class act. He embodied the aspects of the art that he, and the other post war students of the Founder (like Arikawa and Osawa, amongst others) had felt were the aspects of O-Sensei's Aikido that would be of the greatest benefit to the typical modern citizen, both Japanese and foreign. Aikido became, starting with O-sensei and continued by the Nidai Doshu, a transformative art, rather than a fighting art.
It's very difficult for anyone today to discuss Aikido and the differences between O-sensei's view of the art and his son's. There are hardly any folks left who directly experienced the Founder's art when it wasn't highly influenced by his students. Tohei changed how the modern post war generation perceived the art, Kisshomaru was central to defining what the whole post war generation even thought the art was.
Most folks doing Aikido today can not even tell you what "aiki" is. Yes, there is a distinct difference in nuance between "harmony" and "joining" but regardless of which of these definitions people prefer, the vast majority could not really tell you what it means in terms of the physical practice. Whether I am "harmonizing" or "joining" with a partner or opponent, the fact remains that most Aikido folks couldn't tell you, in anything other than the most rudimentary fashion, what either of those terms means. Oh yes, Aikido is the art where you "harmonize" with your partner's energy. Or perhaps you "join" with himů
My own opinion on O-Sensei's great shift in redefining the term "aiki" would be that he took the term from it's traditional usage, which I believe was much closer to "joining" than "harmonizing", and made it into a state of being i.e. "already joined". With that fundamental shift in orientation, one then comes back to "Harmony". Harmony describes the balance of the infinite elements within the whole.
So, technically, harmony is descriptive of what O-Sensei meant when he talked about "aiki" (as in Take Musu Aiki" etc.) The essential problem for the post war generation of Aikido, especially the folks who didn't speak Japanese, was that the term "harmony" is a term loaded with associations. It connotes everybody getting along, it connotes, elements in the system all tooling along happily.
So the folks reading Kisshomaru's writings start understanding Aikido as meaning the "Way of Harmony" and Aikido starts to morph into an art which, at least generally, has no conflict i.e. ineffective attacks, complaint ukes, etc. People start to think that this is fine because they are creating "harmony".
This of course is an imposition of various outside world views onto a term which probably wasn't best translated that way in the first place. Certainly, the Japanese desire for consensus and group cohesion informs this transformation. So do the Western emotional associations with the term harmony as having a connotation of lack of conflict. So we have people seriously maintaining that Aikido is about "conflict resolution" but from whose practice any actual conflict has been removed. Harmony becomes hanging out at a dojo with a bunch of like minded folks who share the same basic interests.
The aspect of "harmony" having to do with the overall balance of the System, not with the fact that the components of the system are constantly changing, colliding, combining, falling apart, coming back together, gets lost in this pursuit of lack of conflict.
Folks blame Kisshomaru for this, undeservedly in my mind. If one takes the view that Aiki can be translated "accommodation to circumstances" as you point out, things make a lot more sense. It acknowledges that circumstances are changing and that or task is to learn how to deal with that. It puts "conflict resolution" largely where it belongs, as a personal endeavor with oneself. And it has direct relationship with practice n the mat. "Accommodation to circumstances" then becomes a physical and mental practice in which one learns to approach change in a non-resistant fashion.
So one can then see the progression from koryu usages as describing "aiki" as a set of perceptual and psychological, techniques used to control an opponent, accurately translated as "joining", to O-Sensei's usage of the term "aiki" as a state of being in which is "connected" with the flavor in translation of "already joined". Then one can see that, for post war teachers like Kissomaru, trying to figure out how the art should be presented to the world in a way that does the most justice to the Founder's goals, the mystical connotations of the term "aiki" when understood as "already joined" presuppose some level of spiritual development that wouldn't generally be present in the "market" to which Aikido is going to be presented. So that use of the term, while the truest to how the Founder himself probably understood it, wasn't going to be beneficial to the large numbers of folks who would be exposed to the art going forward.
So what are left are largely the moral and ethical aspects of how someone with that deep level of understanding would comport himself, or herself, in his everyday life of constantly changing circumstances. Aikido becomes a body based system, still a form of Budo in my opinion, which attempts to impart a set of values, much reduced from the full scope of the spiritually based philosophy of the Founder.
Whether it is performing as conceived is open to debate. Whether it was necessary to pare so much of O-Sensei himself from the art to make it palatable to modern minds and non-Japanese practitioners is also an active debate. I would maintain that too much of O-Sensei was dispensed with in forming the modern presentation of the art. But I do not believe that Kisshomaru and company were wrong about reinterpreting the principles of Aikido from O-Sensei's conceptual paradigm to one better suited for mass consumption. If hey hadn't done so, the vast majority of us wouldn't actually be doing Aikido. The message that was presented as O-Sensei's, although edited heavily, obviously has spoken to people world wide and continues to do so.