“So if someone is mentally ill, is it the mind (spiritual, demonic, etc.) or the translating organ of the brain (age, MS, injury, other disease, etc.) that is the cause, (or perhaps both)? How does one distinguish the causes?”
Many powers of the mind (such as sensation, imagination, appetite, and will) are physical, in the sense that they are powers of the composite of the soul and body. I’m not referring here to strict materialist theories of the mind, which are nonsensical, but to hylemorphic theory, which is a much better grounded philosophical basis from which to start.
Because many mental powers are derived from the body (the composite of matter and form), they have a real physical component. Physical illnesses (injury, intoxication, etc) affect physical processes (sensation, imagination, appetite, will), and cause disorders of these faculties by direct alteration of the brain, which is a matter/form composite.
There is only one aspect of the mind that is entirely immaterial- the intellect, by which we understand, reason, judge, etc. When the intellect is affected by a physical process (e.g. brain damage, alcohol), it is because the material aspects of the mind–sensation, perception, imagination, etc- present the intellect with disordered information. The classical hylemorphic (matter-form) understanding of the mind, which is that it is one of the powers of the soul, which is the form of the living body, accords quite well with the modern neuroscientific inference that alterations of the brain correlate reasonably well with alterations of the mind. The classical understanding also emphasizes the immateriality of the intellect, which I believe is essential to a genuine understanding of the mind.
Because the intellect, unlike the other powers of the mind such as sensation, imagination, etc, is entirely immaterial, a primary disorder of the intellect would be intrinsically immaterial, and might be thought of as a spiritual disorder of character, or a tendency to sin, or even demonic influence/possession.
It should be remembered that neither the mind nor the brain are separate existing substances. What actually exists is the person, who is a composite of form (soul which includes what we call mind) and matter (prime matter- pure potentiality), and in mental illness it is the person, not the mind or brain or spirit understood in isolation, who is ill.