Fwiw I have a (very newbie) perspective on this as a doc (though not psych); my mixed-up MLC H is a physician too. And since this broke, god knows I've seen my share of docs...
Two days after BD - I was working in the ICU, palpitating madly through rounds and generally a sub-clinical basket case (fine timing, H!!) - so called the local docs-at-risk line. First psych I spoke with described MLC as "childhood chickens coming home to roost". But really he attributed the problem to full-blown, life-long narcissism on H's part, sent me off to read about personality disorders, and had me half convinced that I was a complete idiot and doormat for not seeing this 20 years ago. ...Eventually I found another psych. Current doc equates MLC to a mental health crisis, for which depression may be the root cause (and in which narcissism and passive-aggression may appear, but are not previous hallmarks of the MLCer's character). I can live with this definition! The MLCer's behaviours and coping mechanisms are similar to those of an alcoholic; and therefore she recommended the same coping mechs for me as she would for someone living with an alcoholic.
My H is from a dysfunctional, highly judgemental family. He was non-confrontational and therefore avoidant. Surely a number of Ericksonian stages had been left unresolved. But perhaps that laid the grounds for poor coping mechs. I've read about FOO issues from many of your stories, seems to be a theme.
I do think my H's precipitating problem was dysthymia, maybe depression - disturbed sleep, anger, anhedonia, lot of the psychomotor signs. He tried to self-medicate by "chasing the dragon" - more and more risky activity, hunting for endorphins to provide that happy feeling, drown out the emptiness. For the past year, he's been cycling with greater frequency, and if the fix didn't work (no "high", body didn't let him summit the mountain, etc), crashed into rage. Being a doc and therefore pig-headed (!), he refused to consider this might be a mental health issue. Then the crisis....
So generalizing sadly from my little experience, I wonder if many of the risk-taking behaviours in MLC fall into the self-medication category (including OM/OW in this). Brain serotonin deficiency, though critical, seems too global a word; individual neurons may have deficient transmitters, but it's the connections and how they're managed that are important, not just the act of neurotransmission. Too fine a process to turn up on any scan. Antidepressants are like using a mallet to turn a screw (they work sort of, but...). I don't know that there's a good scientific (or pharmacological) handle on this. As usual, the best answers come from psychology, not psychiatry.
"You have a right to action, not to the fruit thereof; shoot your arrow, but do not look to see where it lands." -Bhagavad Gita