"Does pain make someone inoperative or at least tactically inoperative?" "Do all people tactically fail at the same pain tolerance level?" "Can pain tolerance failure be objectively predicted and applied universally?"
It can make someone inoperative but does not always. Vaso-vagal faints for example. It often however makes them more cooperative as the body tends to move away from pain.
No not all people have the same tolerance to pain and it cannot be objectively predicted or applied universally. Some people simply do not respond to pressure points. Some authors have suggested this is about 10% or less.
My position is that atemi to pressure/vital points is:
1. They are a useful adjunct to the aiki principles.
2. They are part of the martial aspect of the art (this is the opinion of several high ranking aikidoka as well).
3. They do not always work on everyone-those with high pain tolerances, deeper nerve structures, on drugs (PCP for example), etc.
4. They can be applied without interruption the flow of the technique or the "aiki". This can be a bit challenging.
5. They are not for everyone-some chose to simply not apply them. I for one believe they are important as an element of the overall technique.