Re: Principles of pinning
Well, for me, a pin is a codified control mechanism, a formal and conclusive ending to kata (so to speak). I could, for example, end ikkyo omote by dislocating uke's shoulder as I descend into seiza. However, The kansetsu waza for ikkyo omote is to pin the arm to the ground. In one of my earlier posts I referred to tools of pinning and in similar sense, pins are tools of control. I do not think I would classify pins differently from controls unless the differentiation was in the classification. A pin is an action to employ a tactic of control within a combat strategy. I could see the claim that a pin is an action, while controlling your partner is a larger tactical decision. Remember, the preservation of combatants is a key strategy in aikido, achieved primarily through grappling and striking; I could also decide another strategy which could change my tactical responses.
As for my perspective, I believe in that the resolution of any engagement is at the instant of aiki. If my partner controls my center first, I lose. This is similar to the concept of ichi go ichi e - I have one chance to succeed in the harmony of aiki. Aikido is a collection of tactical actions encompassed in a philosophy that guides our strategic decisions.
At the core of our tactical actions is an understanding of the unity of aiki that must precede engagement. When done properly aiki not only controls the point of contact or even an appendage - it controls your partner's entire body and cognition. For me, the unification of your partner onto your center is the harmony in aikido because your connection allows for empathy and compassion, true feelings of agape. If you do not have this connection then whatever it is you are doing, it is not aikido.
Kansetsu waza is an expression of that control. Arguably, if you control your partner's entire body balance from the beginning, how hard can it be to simply transfer that control into an arm, or a leg, or a neck? I work out with guys who will touch you and put pressure into your back foot, then your front knee, then your right elbow. They can do this because they controlled my body at the point of engagement. In the best aikido I experience my partner has control of me the entire duration of our engagement, not just when she twists my arm, or yanks my neck, or locks my elbow. So it is in this state of aiki that I am either wholly victorious or not. If I need to contest the engagement, then that is proof that I did not achieve victory at the point of aiki and therefore we are fighting.
In a larger sense, I think that this perspective is not necessarily practiced en masse, although I know many people who intellectually understand the concept. I believe this is [one of the reasons] why Gleason sensei [correctly] observed that we are no longer practicing "aiki". I believe this is why Ikeda sensei altered his approach to explaining aiki, I believe this is why Ledyard Sensei advocates a strong focus on aiki before teaching waza. This is why Saotome Sensei is working with other organizations and creating bridges to find better ways to explain what we are doing and why. These are people I respect and I believe are making great strides to bring this education back into aikido here in the US (and I apologize, there are others but I am not as familiar with them).
I am not sure if this answered your questions, but I am working through some of these answers myself.