Re: The shoulder grab - where does this come from
I would have to really look, but I know with several techniques in Budo Renshu, he said if Uke was pulling or pushing, I would guess some of these guide lines were from Kata or Sode attacks as well.
I actually added up all the technique attacks in Budo Renshu last night. The book heavily is weighted towards Soda, then Kata dori techniques. It was clearly important to him.
The desire for Uke to keep holding is something that we see in lot's of Aikido techniques. This can easily be explained be the need to keep a hold of a weapon. I've used (in a Dojo) a sleeve grab many many times to keep an armed person from "stabbing" me. It's useful. But beyond this, I was doing some ground grappling last night, and realized that lot's of times I would hold fast to the gi sleeve, even with out the motivation of a weapon, simply because the sleeve grab gave me some control.
Anyways, those are my thoughts.