>>16843110 (OP)
>Is the fundamental definition of "mental illness" no more complicated as "anything the government wants to define as an illegal belief"?
It would be more accurate to say that a mental illness could be defined as anything that compromises the survival of an individual within a group or environment (both social survival and biological survival). To this extent the definition of what constitutes a mental illness can vary, yes.
To contextualize this: nowadays schizophrenia and epilepsy are seen as disruptive and scary, but in the past (and even today in some 3rd world shitholes) these mental disorders were seen as a qualities necessary to be a good shaman. The ability to see shit that wasn't there and the ability to enter into an epileptic seizure or fit, were both perceived as supernatural abilities benefiting a person who was expected to interact with magic.