Dear friends,
I had my confirmation today that indeed his 'dark period' has nothing to do with me or our relationship, so I can definitely put my mind at ease on the matter.
I called a friend at the farm to catch up and he told me that my ex left suddenly this morning with no warning and without even saying where to (later, said ex wrote to tell me he's moving above a bar we used to go to where he'll work a few days a week) and that he thinks the ex is getting worse and worse.

I have to admit part of me still hoped that after I left, he might feel better - it would have stung, of course, but at least I'd have known he was okay. I'd say all your wise words about how his issues, whatever they might be, really had/have nothing to do with me have been validated. I'm really, really sorry for him, but I hope it doesn't make me a horrible person to say that having this confirmation is going to help me greatly in peacefully moving on from what happened.