Kotodama
I, too, have long been interested in the kotodama and feel that since it was important to O-sensei, that we should try to understand it a bit. It is a very esoteric subject and derives (need some history here professors) from ancient practices, probably India. It starts with the simple premise that, "in the beginning, there was the Word." "U" or "SU" or "AUM" or some such is the basic sound that generated the entire Universe and all phenomena exist bacause of the various vibrations, etc., etc. This is in fact, not so much different from our current understanding of the Universe derived from scientific research such as physics, mathematics and astromomy. We can say simplistically, that all things are vibrations.
That being said, the theories of kotodama are complex but are probably not linked to a specific language. I think that you can chant in any language you want and get the same results, provided that the sounds you used were the appropriate ones. There may be theories about how to combine sounds, how to heal with sound, kill, catch fish, who knows.
The bottom line is that it is a fascinating area for further study, however, much scepticism must be exercised. There are no legitimate Aikido dojos that I know of that teach kotodama. O-sensei talked a lot about specific sounds as related to techniques (from what I have gathered from conversations with some shihans)but he did not teach kotodama. Anyone who says that they are teaching O-sensei's kotodama is BS'ing.
I do not know where you could get legitimate instruction, but I do know a couple of folks that you should stay away from. I think that if you are curious, the little bit of written information cited above will be a good starting point. Then just sit in seiza and make sounds for a few years. In answer to one of your questions, the kotodama deals with the fundamental mystery of the Universe. I think O-sensei would have said the the kotodama was manifested in a technique, not that sounds would aid breathing or make the technique work better. If you have faith and train correctly, perhaps the kami will favor you as they did O-sensei. I mean that sincerely, not in jest. Good luck.
Ian McDonald
Ichi go, ichi e
The swamps are good along the WeekiWachee