As for all your "why's"--try checking out the results of low seratonin, which is what happens during depression, on the human mind. Essentially, the depressed person's brain works quite differently. Things that seem to make no sense to non-depressed people almost seem to be the norm to them--at least, so many of them act and speak in almost predictable ways. Is it really worth going over and over the "why's?" Until his brain becomes un-depressed, he's not going to be able to see things from your perspective, period. So try not to get stuck on that; take care of your own happiness instead.
You asked about timelines in my H's depression/MLC. This is what it looked like: His mother died suddenly and he entered denial--a time of numbness, passive aggression, when he seemed quite shut off from me and the rest of the world. Then came the stage when he was angry about everything: work, the news, my family, the kids, where I flossed my teeth, etc etc. These 2 stages together lasted about 13 months, though the anger could flare up for another 2 years.
Next came his replay stage, in which he neglected the house and his children, and focused on new (younger) friends, new music, new clothes, more alcohol, talked about getting a new job or moving or getting a sportscar (which, out of nowhere, he'd suddenly "always wanted"). He completely changed his history so that I'd forced what had been mutual decisions on him. Then, of course, he met the EA, and she became the focus of his thoughts. He talked about how he'd always had to be responsible, but "now it's time for me." He was always tired and achey, and spent all his time either away "for work" or on the sofa, complaining the kids wouldn't let him sleep. This stage took another 2 years.
His "awakening" came on the day he could finally admit that he had been lying to me about the extent of his connection to the OW. He also began cycling--one day he could see that she wasn't "just a friend," but a threat to our marriage. The next day he'd ask, "what's wrong with having a good friend anyhow?" It was scary to listen to him: he clearly had 2 minds (the teenaged and the mature) but was unable to control which one dominated. On a day when he thought he wanted to choose me over her, he said good-bye to the EA.
Immediately, he slipped into the depression stage. He thought a lot about the agony of not having EA in his life anymore. He'd lie around sighing deeply at night, admitted to suicidal thoughts, had virtually no sleep, stared vacantly through alien eyes, and occasionally hugged me as though I were a wooden post. He was completely oblivious to my pain, just asking "why did all this have to happen to me?" and feeling like a victim.
Then came the withdrawal stage, during which he rarely communicated about his thought processes to me (although he spilled his guts to all sorts of inappropriate people). He renewed his contact with the OW, but felt terrible guilt each time. He felt more aches and pains than ever before. Because he was seeing a counsellor thorugh this time, these 2 stages lasted about 9 months together.
Finally, he said he wanted to work on the marriage. He said goodbye to the OW (who he still saw as virtually perfect), and took an interest in the kids again, and stopped feeling such pains. Of course, I expected he'd apologize, and recognize my strength in getting through this nightmare, and be his loving self again. Wrong!! Coming out of the depression took another 9 months. He didn't get me anything for our anniversary--though he'd put a lot of thought into it--because he couldn't make a decision. He seemed to stop all attempts to improve things between us. For the longest time he would shut down if I said anything about his whole MLC and EA, or even if I was obviously upset about anything. He'd occasionally say "sorry," but couldn't explain which part he was sorry for.
Gradually, he became his happy, positive self again. We can talk about pretty much anything now, and he can even laugh about the past. But we could never talk about it while it was more "immediate," and as a result, most of what I would have wanted him to know never got said. And that's probably all right, because it's not as if he can remember a lot about his time in depression.
In total, it took H 4.5 years from the time his mother died until he felt happy and in love again. Without his C I think at least a year could have been added to that. Your BF looks like he still has several stages to go through--the infatuation with the OW usually lasts about 2 years, for one thing. Until he has completely come out of the depression, he won't understand your pain, frustration, anger, etc, so you're better off dealing with those on your own, as in Passionate Marriage.
As an aside, I'm very pleased to hear that your BMF is choosing to end your relationship . That really is the only chance his marriage has, and I hope you will be a big enough person to break the connection, for the sake of his wife and child.
