I just had a thought. A person who was psychotic, schizophrenic, or otherwise mentally ill could very easily be thought to be demon possessed to a primitive culture. Indeed, I thought I was demon possessed for a long time. I actually asked my pastor if he could perform an exorcism on me once.
Legion, from the Bible, was said to be demon possessed. He exhibited many of the symptoms of schizophrenia. Screaming, self-inflicting wounds, convulsions, among others. Augmented strength and enhanced tolerance to pain (both of which Legion was said to possess) are not uncommon during these kinds of episodes. Indeed, I have inflicted wounds on myself that I didn't feel for hours. (It's really complicated as to why. In a nutshell, the self-inflicted pain is easier to deal with than the mental pain)
I was just thinking that our verser who is active in medical study could encounter a demon possessed indig, and find that he is really suffering from some form of mental disorder. It was just a strange thought that I had.
Or conversely, he could encounter a "demon-possessed" indig, assume that he must be suffering from a mental disorder, only to later discover the indig is really truly possessed by a demon. This one is more effective if the indig is in a hospital or some other institution of medical care when the verser runs across him.
This is stuff like in Farmland which has an Alpha and a Beta. In one world, if you show signs of being weird (hi-tech, martial arts, magic...) they scream 'witch' and reach for the kindling. In the other, apparently identical world, you have to show signs of being weird because high-tech aliens are invading the world and if you want to defend it against them you need something a bit more than a sword or a pitchfork.
As a technical aside, Legion was the name of the horde of demons possessing a man whose name is not given. Otherwise your analysis is fairly accurate.
It is certainly not easy to determine the difference between mental illness and demon possession--as your own experience suggests. And yes, since there are skills for dealing with spirits as well as skills for dealing with mind and brain disorders, both must exist in the multiverse, and some diagnosis must be required to determine which is happening in a specific instance.
On self-inflicted wounds, I'm sure I've injured myself and been unaware of it many times--I often wonder where I got bruises and cuts. In my case it's my level of mental distraction at work, that I crash into things and keep going because I'm not really attentive to my environment (I tend to live in my head more than the world around me). So that's not so uncommon and does not really require either serious mental illness or demon possession. Of course, doing it intentionally is definitively mental illness, as the medical profession defines self-destructive behavior as mentally unsound. This is probably an oversimplification, since doctors engage in "destructive" behavior such as the excision of body parts (e.g., infected gall bladder, gangrenous foot) in a "lesser of evils" mindset, and it is not at all unreasonable for a person to make a similar choice for himself. The medical professional would then challenge whether the individual was in touch with reality--if you remove your hand because you believe a demon was controling it and trying to kill you, a doctor who does not believe that demons can do this is going to believe you are mentally unstable and a danger to yourself, at least (but you might have been right?). So the choice to "hurt" yourself might not always be crazy. Before anaesthesia, surgeons gave patients something on which to bite to focus their attention away from the pain, but there are techniques which introduce a second pain in the body to distract the mind from the primary pain, and sometimes a lesser pain in one place can reduce the impact of a greater pain in another. Pressure points work much like this.
I tend to think, though, that for most versers mental illness would be the first guess, and demon possession an afterthought, so I'd probably choose Scott's scenario first.
--M. J. Young