I agree with you that there is definitely some serious obfuscation around the chemical imbalance aspect of the biological theory approach to mental disorder. If you do any research into the history of anti-depressants it isn't hard to uncover that there has been a lot of questionable science that forms the foundation of the over prescribing of medications for mental disorder that is currently the industry norm.
I spent years suffering with really severe symptoms and I was so desperate for relief I tried all sorts of treatments. The main thing I really learned during that time is that approaching mental health requires a holistic approach, but even armed with that knowledge my life was still seriously impacted by my mental disorder. For me, I had spent years in talk therapy and different behavioral therapies. I had also tried various medications which did not help me and/or had side effects that were unbearable long term. I was on lamictal for a while which seemed to help but I also stopped seeing improvements and eventually that was also not good enough and my function declined. I also tried meditation, spiritualism, self medication, yoga, exercise, different diets, journaling. I took class after class on psychology and I instituted all sorts of different systems to try to fix myself. My therapist told me over and over that I was doing the work but that the improvements we were seeing weren't what she would expect, then I was referred to TMS. Once again, I was desperate and I was willing to do or try pretty much anything because I was a wreck of a person that was spiraling dangerously. So I went, and for the first time about three weeks in... I started to see a small difference. It changed my life, and I'm still so much better. The thing about how well it worked for me though is this: the idea of TMS is that it targets the part of your brain that is damaged by depression. Whatever this means- I've come to imagine it as neurons that have formed hard wired patterns of depression that are nearly impossible to rewire through sheer force of will.
So chemical imbalance- probably (even demonstrably)false. Physical patterns in the brain formed while you suffered from depression being very difficult to rewire without outer interference? Is that possible? I'm mostly thinking out loud and I don't really expect someone to just have the answer but I really want to know and it's something I wonder about constantly.