Damocles, I hear ya! Cause and effect or effect and cause? I can relate some of my first hand knowledge that may shed a little light on the problem.
I'm 60 years old, and as any type 1 bipolar sufferer will tell you, over the years we will experience a few hospital stays in the psych unit. Although what I'm about to relate is anecdotal, it is my real experience. First of all, I was suffering from BP 1 long before I became one of JW's. (I'll revisit that in just a moment)
Almost EVERYONE who ends up in a hospital is in very bad shape. Here in California, either you, or your psychiatrist, or the police may present you to the emergency department on what is called a 5150, which in short says, "you are going to harm yourself or others". So the point is that living with mentally ill people who are suffering from the worst of their illness! I was curious about how "religious" the other patients were. This curiosity came about when I heard 2 patients arguing about their personal choice of religion. I casually asked as many of the patients about their stance on religion, that would answer. Big surprise to me was it was difficult to find anyone who wasn't religious in one sense or another.
Disclaimer: The following is my take on what I had heard.
One basic fact one needs to remember is that almost NO mental illness comes in a "pure" form. IE depression could comes with an anxiety disorder or schizophrenia with delusion etc. Almost all of mental illnesses create a certain amount of "Chaos" in our minds. One thing the human brain hates almost universally IS this chaos. (we find patterns where none exist, like cloud formations or the stars) Since this disorder of the mind is in many ways to much to take, we will do just about anything to put more and more structure in our minds and the more rigid the better. Just about all religion, with it's trappings, will give some relief from the chaos and more structure the better. It doesn't surprise me that some mentally ill people will "like" the structure of being one of JWs. But my experiences have also taught me that there are more mentally ill people in ANY highly structured group. Again, this is only my conclusion based on my years of experience while getting to know others with mental illnesses. (sorry for the long post)