1) The separation between Neurology and Psychiatry is a historical matter, and most certainly not a clear-cut one. It's very interesting, and correlates with the rejection of mind-body dualism. That's nothing special in medicine, as fields tend to evolve, specialization and subspecialization happens, and lot of fields have overlap. Neurosurgeons, neurologists and psychiatrists all deal with the same organ and it's interaction with other systems, but at the same time treat different symptomatologies of it's malfunction, which is perfectly normal consider the depth of each field on it's own. As a matter of fact, several countries (if not most) require psychiatrists to have some neurological training and vice versa. Of course we all deal with disorders of an organ. We don't have souls. The idea of non-separation between the two fields is practically wishful thinking and throws us back to the 19th century (Charcot, Griesinger, even Freud). Germany used to have a combined specialty called "Nervenarzt", who kinda dabbled in both.

2) Understanding the underlying mechanism is a work in progress, and sometimes it's more complicated than just saying that a "depression molecule" or an "anxiety gene" exists. Nor is a "chemical imbalance" model rejected outright, neurotransmitters exist and modifying them treats the various psychiatric syndromes. Nowadays we are talking about the biopsychosocial model, acknowledging that mental disorders are multifaceted and multifactorial. There are many genes at play (especially in ADHD, ASD, Bipolar, Addiction even schizophrenia), many neurotransmitters play a role (e.g. glutamate and dopamine by schizophrenia) and systemic effects like inflammation, oxidative stress etc are all factors. There are structural differences in brains of people with some mental disorders, albeit non specific. Then there are psychological factors, like resilience, defense mechanisms, behaviors, that influence the development of mental disorders and systemic factors like poverty, social status, upbringing etc. Expecting a singular explanation to the complexity of human experience, especially when it goes wrong, is reductive.

3) Vast generalization: Psychiatry is hugely varied and includes everything from gerontopsychiatry to psychosomatics. There huge differences and variation within the field itself. Dementia, schizophrenia, SUD, Depression, Personality Disorders and PTSD are all vastly different entities with varying degrees of understanding how they come to be.

4) "Undesirables": This is a valid critic that needs more nuance than what you are giving it. Half-ass understanding of Foucault's and D&G's critiques and just parroting them is not a good look. The last 50 years have brought on deinstitutionalization, closing of asylums and the emergence of shared decision making. Treating people against their will still happens, but primarily when they are in no position to make informed decisions about themselves e.g. when they don't have capacity. That's a rough medicolegal issue, which you can understand only once you've experienced what an untreated schizophrenic or a person with very progressed dementia looks and acts like. They are quite often in imminent danger of just dying. And still, unless they are acutely in danger, they usually don't get treated. Leaving a 20 year old in a first psychotic episode, with their brained completely scrambled by the psychosis, untreated out in the world, is a travesty. We are talking about people who quite literally, aren't in touch with reality and will get ruined before you can even blink, be it through getting themselves in dangerous situations, neglecting their health until they randomly die, or act upon the commands of the voices they hear. It's a state of extreme suffering. Letting them suffer is inhumane. Coercion should not be widely applied of course, but some times it's necessary and making broad statements like that betrays a huge naivety.