I have just read your web site and found it to be both informative and thought-provoking. Currently, I am doing a report for my psychology class on the middle ages and the churches and how they treated those that were afflicted with mental illness. I have taken the position that the Roman Catholic Church during that time accused people of being "possessed by demons" when they wished to gain in power or coin or even land holdings or to disgrace or publicly humiliate that person or family. I believe the Roman Catholic Church abused its position to force congregations into accepting its practices and beliefs through threats of death and torture under the guise of accusations of possession which many were burned at the stake or tortured in other ways. However, I am having difficulty locating reference material to support my position and beliefs for my paper. I was wondering if perhaps you might have such material that I could cite and use or know of additional sites that can be used. My paper will be viewed by those in my college and read by peer groups.
Your theory sounds fascinating. It is easy to imagine people in power abusing their position and using false accusations to reach their goals. The Roman Catholic Church does keep extensive archives, but I'm not familiar with how you would be able to access them. Given that your paper would caste the Roman Catholic Church in a bad light if proven true, I suspect that its members would be reluctant to aid your research.
I can show from Scripture that demon-possession while existing at the time of Christ and the apostles, was promised to end. (See "Demoniacs" for details.) Therefore, by extension, any claim of demon possession after this period of time would necessarily be false. This would leave two basic causes for accusation: ignorance or punishment. The former would be attempted by people to explain what was, to them, inexplicable. The later is the basis of your research.
The tough thing would be attempting to second guess the conclusions so far after the fact. It is not that you can interview people or perform diagnostic tests. The best you could hope for is an accurate description of the person's symptoms to see if they approximate a modern medical diagnosis.
You could make a rough separation between accusations of abuse or ignorance by examining the consequences. An accusation that just "happened" to lead to a benefit to the accuser or a friend of the accuser would be highly suspicious.
One book that documents the abuses by the Roman Catholic Church during the Middle Ages is A World Lit Only by Fire by William Manchester. Look up "witchcraft" in the index. The information is not precisely what you are looking for, though it is included in the general discussion. The book focuses more on abuses of power in the leadership, such as among the popes and cardinals.
In addition to searching out demon possession, exorcism, and witchcraft in the Middle Ages, take a look at references to asylums. You might find interesting comments there. The Catholic Encyclopedia has an interesting article titled "Demonical Possession" that lists several cases from the Middle Ages. They could be tracked down for more details.
Good luck with your research and paper.