CK may not have been "martial", but it definitely was more that a "cleansing" or "purification" ritual.
Probably... but without some sort of context or reference, cultural or otherwise, we're just guessing. If I am correct, the chinkon rite of purification is essentially a funeral rite in which the departed souls are "pacified". It is closely associated with tama furi
(shaking the spirits) - which is used to invocate the departed soul of the dead or to energize a weakened spirit (as in someone on death's bed - I think). As for there being more to it than meets the eye.... probably... BUT unless one has been initiated, immersed in or and is an indigene of the culture, one is merely engaging in "external and meaningless" ritual movements.
In terms of "possession", I've been introduced to some old Chinese qigongs that also involve "possession", but what they do is establish a relationship between "automatic movement" (as in automatic writing, pendulum swinging, etc.) and the essential part of ki/qi. There are some offshoot groups/cults in China that also go into the same sort of "possession", but without going into it, I can see a possibly strong relationship to O-Sensei's view.
Granted, but for most westerners brought up in a Christian world view, the blend of Shinto, Buddhist, and Neo-Confucianist beliefs of Japanese society is largely a culturally foreign concept. My understanding is that this philosophical/belief system forms the basis of many koryu doctrinal teachings.
I recall some bersilat practitioners engaged in similar practices involving spirits, demons and trances...
Speaking of possession... reminds me of my Confirmation... here I was, a young lad of 14, watching the priest perform the "scared ritual" and half expecting his invocation of the Holy Spirit to suddenly enter my body. How disappointed I was, as I slowly watched the priest mumble some incoherent latin as he walked past me.... and.... I felt... nothing. Absolutely... Nothing. Either the priest was bogus or I didn't believe enough. I'm leaning towards bogus...
But all this talk of mind-bending, head twisting, demonic possession and green ki projectile vomiting makes me think of that poor little girl Carrie...