I would say yin and yang come from one point. They are one, inside and outside.
Anything in me exists before between me and thee so obviously that would apply to Aiki or anything else so I fail to see any other significance to the statement if there is any.
Don't know what the significance of bouncing off is apart from a skill based on stillness. Hardly all embracing Ai ki as I see it. Part of maybe but wholly? No, not to me.
Valuable? Yes. Limited as far as Aikido potential? Depends solely on your intention and purpose.
There really is no debate. The one point model for yin and yang is severely limiting. This is -of course- why Ueshiba hardly discussed it at all. The things he actually discussed were far more advanced and more difficult to do.
As for power and bouncing off "releasing the mountain echo?" Well...why was it so desperately significant....that Ueshiba continued to practice it and demonstrate the very thing you just dismissed....... his entire life.
As I said there really is no debate; he did what he did, he said what he said, and it all fits nicely into the same model that has been written and passed down for ages.
I guess another interesting debate point is who can do what he did? And to what degree of success?
Maybe the ones who have more of his power and softness are the ones who understand more of his message.