I'm still online, so let me try & answer your questions.
1. My H is a living, breathing, walking example of depression. He has all the symptoms, and has for a long time. My own counselor told me almost 2 years ago that he believed 90% of what was happening in our M (EA to PA, moving out, acting out, mean talk, confusion, etc.) was my H's attempt to grapple with symptoms of depression. I also had a thyroid problem for a few years and I think that exacerbated some of the things he was feeling... I wasn't easy to be around sometimes and he took this personally.
2. My H has known for a long time that something is wrong. He briefly went to a C who was of no help (I strongly believe he needs goal-oriented counseling, not a plunge into his past). He first identified it to me as depression almost a year ago. I don't think he's known how to deal with it since then. I do think he has come to see that removing me didn't remove the feelings of depression. In fact, I'd say it's much, much worse since he left home.
3. I think what pushed him to seek help (and this is speculation based on what little he is saying) is that he is at the bottom of the hole he flung himself into and he has repeatedly been trying to crawl out. His sleeping patterns are all messed up, he is missing appts. with people, he is feeling extremely low and hopeless. His daily life is suffering (and so are his efforts to reconnect with me).
4. What I did woefully wrong at first: told him everything my C suggested about his mental health and his A, sent him articles, got angry at his choices, cried, told him he was very screwed up one day out of anger, etc.
4. What I did right: realized it wasn't about me, stopped making things worse by doing the above. Apologized for my previous reactions and words, and continue to let him know when I'm wrong. Let up pressuring him. Offered him support and suggested he lean on his best friends if not me. Chose to not let his depression and negativity push my buttons anymore. Detached from it. Offered him health insurance. Showed appreciation when he made steps toward his M.
I do think there's some MLC here. Definite possibility, since he's followed the pattern for it fairly closely. Also, his dissatisfaction has gone beyond our M to his work, etc. too.
