I want to add (had to leave to get somewhere...) that it is one thing to talk about metaphors, as in "oh, he must have meant....harmony" but in application this will not work - it is the actual things that are yin and yang that count.
A TCM practitioner cannot provide proper herbs and acupuncture on a metaphor of "harmony" - he needs to know what specifically is manifesting yin and what specifically is manifesting yang and how to guage which in and of itself is deficient or in a surplus and THEN figure out how to restore harmony.
If I understand Dan correctly - and I may be off base as we've still not met in real life! - in the martial arts application yin and yang similiarly is NOT a metaphor for an ideal state of harmony but actual things in their own right to be reckoned with.
Aiki in yo ho is a means to create and retain a balance of yin and yang within yourself and then how it is manifest in meeting force. It is from there, in accord with the three origins: Heaven/Earth/Man, and self rotation, and spiral energy that we manifest "The eight powers." from in yo.
Metaphor is interesting in that in the classical or traditional sense it was always carried forth in scrolls or in poems (my sig line comes from a chinese classic poem), combined with hands on. Neither mode, in isolation, will suffice for true understanding. What I think is certain is that knowing the body parts is almost useless without the hands on. I think a whole lot of people are just kidding themselves.
In regards to his writings, I think most should just "Sit this one out." We've been there and done that. Except for Shirata, O sensei's deshi haven't come up with anything meaningful in the past that I am aware of. All we've gotten for their efforts was misstranslations and etherial nonsense or external power. Because of their own inability to understand (for whatever reason) the rest of us haven't even begun to scratch the surface of what the art truly meant to Ueshiba in his own words.
"I think that if you cannot understand Ichirei Shikon Sangen Hachiriki, breath (kokyu) and Aiki, then even if you practice Aikido the true power of Aikido will not come forth."
How far back do we go to find the sources who held everyone else back, particularly the foreign students? Who knows.