I am sorry to interrupt your discussion.
But since nobody more competent stepped in, it is important to me to comment on the mention of Endo sensei.
Aikido is about joining and blending and avoiding.
I don't think that these words are usefull, to describe the aikidō of Endo sensei. And in fact he himself does not use "joining" and "blending" to describe what he does, as far as I see.
He indeed uses "to avoid" sometimes. But it is meant in a different way: It does not refer to evading a movement of uke. Evading is clearly forbidden in sensei saikidō. To avoid in terms of sensei means to absorb (= avoid) the incoming power
of uke when contact is made. This is done using one's own body structure.
Endo does not teach a lot about what to do (joining, blending, harmonizing ... ) with uke. He teaches about using, organizing, structuring one's own
body. This to him is most important in aikidō. This is what aikidō is about
: Learning to organize one's own body and mind.
And waza are possible ways to applie this to an opponent. He clearly distinguishes the "formal waza", which we cal kata or kihon and which can be named. And the waza that emerge spontanouesly when just using one's own body structure, one own movement. uke is "just" someone who helps tori to explore his own body and learn to use it. (This is also important to understand the way of ukemi: Like in a push test your partner is not an attacker, he is not when practicing aikidō in this way.)
To better understand the way Endo thinks about organizing, structuring one's body it is usefull to read about daoistic body work, like qi gong or tai chi or pakua zang. Endo understands his practice as the practice of dao. And he uses the daoist term, expressions, images, to explains what he does. There are strong connections to the chinese internal arts to be found in his teaching and I experienced it to be very important to get familiar with daoist thinking to understand him right.
When it comes to using one's body structure, Endo's aikidō is not about blending, joining, harmonizing, but about meeting the opponent. This is what is contained in the word "atari" (当たり, compare atemi). This video shows a very basic excercise
to study Endo's understanding of atari. And I think you may get a glimpse of the relationsship between tori and uke in his aikidō. In my eyes this is clearly different from what most aikidōka understand when they talk about blending, joining, harmonizing with ukes movement, isn't it?
Creating atari (= meeting the oppenent) is then used to guide him and to create kuzushi. In this way of aikidō uke is moved by tori. tori does not only controll his own body, but also the body of uke.
You talked about using the shoulders: Well Endo does not use the shoulders or the arms. He instead uses the shoulderblades. This creates a specific feeling and specific possibilities. While the shoulderblades are connected to the spine and the lower body they can move the shoulder in a wayt they seem to be "isolated" or free from the rest of the body. (When you look at the video it sometimes seems that Endo's arms are kind of stiff or muscled. They are not. His arms are competely soft. But there is his lower body "in his arms" via the shoulder blades. Very interesting feeling! You don't have his arms. They are completely relaxed. But at the same time very very strong.)
Again: Sorry for interrupting!