Lee, that smells fishy to me. I dont think there is anything inherent in IS/IP training that would necessarily cause a psychotic break in an otherwise healthy individual. And I sure as spit dont think evil spirits are waiting to jump in if I exhale a certain way.
Way more likely that over the fullness of time, a couple folk who were already mentally ill did some training that became a focus or a trigger for their issues. Anything can do that, aikido, tennis, burgers at McDonalds, an unfortunately timed fart.... Also possible, if Phi is correct about reverse breathing raising BP, some folks may have just had strokes which can certainly change a person's mentation. No demons required.
Your caginess doesn't lend your claim any credibility at all.
I have experienced something like this. A student in our dojo was spending a lot of time studying intensely a qigong system with a well-known teacher, and I started to notice something strange with his body. It's hard to put into words, but to me he felt "disconnected" throughout his body in a way I had never noticed in anyone else, and this was not helping him understand the way I have been practising myself and trying to teach aikido in recent years, which is very much in the opposite direction. At the time I assumed it was his other practice causing this "wrongness" (which is how it seemed to me), and I gently questioned him about it, but he had confidence that he was going in the right direction for him.
Anyway, during one of his visits to the US to see his teacher he had some kind of crisis that involved him going to hospital. This was subsequently diagnosed as the first of several episodes of clinical schizophrenia, for which he is still under medication. I don't know what connection there was between what he was practising and his illness, but I would be very surprised if there were none. He has admitted to me since that he had indeed some feeling that this was the case, and doesn't practise these qigong forms any more.
I am sure that qigong itself is not intrinsically to blame (I used to practise it myself a while back, and found it calming and invigorating). Rather it seems to have been a combination of an particular individual following a particular teacher at a certain unfortunate time of life, and the intensity of his rather inward-forcused exercises may have brought on the crisis.