I'm amazed and pleased by how you had your "crucible moment," and how it gave you direction! Mine also made me reconsider exactly what I wanted my life & any future M to look like. During that crucible day, I looked deep into myself to determine whether I still loved my H (it was 3 years into his MLC and I really thought I didn't any more), to discover that I did, which made the rest possible. But I get the feeling that whether/not you love your H is not a question you can answer yet?
As for his letter, he seems to list all the key points for depression: the depressed person feels that nothing will get better and that he's powerless to make improvements; feels too exhausted to tackle or accomplish change; feels guilty, worthless and self-critical; can't focus, make decisions or remember things. Which is pretty much the hell your H describes.
I think the fact that your H was even able to give you this window into how he feels is amazing--it took a lot just to write the letter. And he did it for YOU--I'm sure he never revealed a fraction of himself to OW like that. I hope he will be able to do IC, for both your sakes, to learn to move past this depression. Many depressed men used work as the crutch which keeps them functioning--resolving the depression might also allow him to create a better family/work balance.
