I dunno, Alameda. What you say is fascinating and I think there's truth in it. (Majority of my bi-polar friends had horrendous episodes in their childhoods.)
But my own experience is that the chemical imbalance suggestion fits best. I've had depression since childhood. For agonising years upon years I cringed in various therapists rooms while they overplayed the importance of the tiniest misdemeanours of my parents or boyfriend as causes of my illness. I tried SSRIs and they didn't work. They stifled the depression and everything else too. I thought I was destined to an entire life of this battle. It felt like walking uphill into a wind that pushes you backwards, forever.
Then I hit a really bad phase with PND which I tried to self-manage for four years with very little success (surprise) I went to the doctor and tried a different SSRI. It was like someone had waved a wand over my life. I felt happy and normal, energetic and capable. And I've felt that way for the whole time I've been on this medicine. If something bad happens I feel sad. If something really awful happens I feel low and cry. Just like people who don't have depression. After time I start to feel fine again. Nothing is out of proportion any more. Sad events don't trigger a vertiginous spiral. I don't ever feel sad for no reason. I feel like I owe my life to the SSRI I'm on now.
One of my DC is low by nature. I've done everything I can think of to teach him positive thinking, to feel good about himself, and he is certainly deeply loved in a stable, secure and very happy family. But I keep my eye out because I think he might have inherited the illness from me. He and I both know we can feel deeply sad for no reason, or bad about life even when it's good. No one has shoved him aside socially or personally. He's just wired that way.
None of what I am saying is meant to discredit what you're saying. I just think there is a vast range of reasons why people have mental illness. Chemical imbalance is, IMO, one. Social pressures and life experiences are certainly two others.
As you pointed out, not nearly enough research is funded into the vast range of illnesses known as mental health. Imagine if research into 'physical health' was lumped together in that way.