Re: Kokyu Nage techniques do they work?
As a datapoint, I spent the past week or so teaching various kokyunage from shomenuchi. We probably covered 25+ different variations. None of them required an uke with a "willingness to fall" in order to work.
It is true that there are some kokyu nage from grabbing attacks that require uke to give a committed grab [in the dojo] for you to be able to practice them.
However, in a practical scenario, if the attacker begins to grab but does not commit to the grab or senses nage's movement and lets go that's fine. As nage begins to move to achieve kuzushi and uke lets go, nage should be in a safe position with good posture and be ready to deal with uke's continuing attack. I would call this a success if nage didn't actually get grabbed. There doesn't have to be a throw and nage shouldn't be concerned with/fixated on any particular outcome.
Back to training *basic* kata, uke is expected to always give a committed attack be it grab, kick, punch, or strike. At a more advanced level, I like to work with uncommitted attacks as well (i.e. feints, setups, etc.). I also like to practice cases where nage initiates the physical portion of the encounter and basically takes advantage of uke's reaction to throw/lock/pin/strike/choke him/her.
I hope this helps answer your question......
In aiki spirit,