I read those books as a teen too. Never swayed my belief in mental illness.
The body breaks down. Or is even born broken at times. The brain is physical, technically. So if the rest of the body can break down or be born broken why can't the brain, and the mind connected to it?
Brain scans show that certain areas of the brain light up, or don't light up, depending on the issue. Chemicals can be measured in the brain/body. These are all consistent with certain mental illnesses.
I have depression. Had it most of my life. Certain situations it is not as bad, other situations make it worse. That sounds, to me, consistent with brain function.
I'm not sure what the big mystery is. As stated above, if the rest of the physical body can be unhealthy, why can't the brain as well?
Even conditions like schizophrenia can change according to circumstances. For example, countries who are more poor and community oriented have a higher rate of recovery for schizophrenics that more 'civilized' countries, such as the US. This is due to how the schizo is viewed and treated by the world around them. They are given the same expectations as other citizens, expected to help out in the village, and have a role. The mental illness is downplayed and they are treated normally.
In the US and similar countries schizos are singled out, labeled and treated differently. While also having less expectations on them. And as a result their illness is zeroed in on, and as a result they don't get past it. So if how a person is treated and viewed changes their mental makeup, then why would that affect a spiritual issue, if that's all it were?
Really thinking mental illness through, there is a lot of logic to support that the brain is ill, not the spirit.