I understand the brain is made of many chemicals and there are imbalances, but could it be that the demonic world, as well as negative influences in life (bad parents, bad peers etc.) could play on those imbalances?
I don't believe that mental issues have any supernatural undertone. Any mental health challenge is traceable to a psychological problem. Trying to spiritualize mental health issues would make it difficult to handle and manage.
Most of the time the cases of supernatural influence (or demonic possession, to be precise) is nothing but hallucination or some kind of mental diseases. One of my relatives had this experience. His symptoms were interpreted as demonic possession and was taken to a shaman but his condition did not improve. One day he read an article from a psychotherapist and was encouraged to visit him. He seem to have been suffering with depression.
People that live in climes that religion is taken to the extreme always want to spiritualize everything. Depression and mental health is not a spiritual thing. It doesn't need prayers. Trying to pray for a person depressed because of poverty might just heighten the entire situation.