"So, Job, where do I find that patience? Is this a lost cause? Should I just cash in my chips? I can do that and I know I'll be fine. But, am I cashing in too soon?"
Where to you find the patience? You dig deeper within your soul. If you have the patience to deal w/your BIL, then you've got more patience to help you w/your h.
Is this a lost cause? "Should I just cash in my chips?" Only you can decide whether to cash in your chips.
Since you are asking these questions, then you aren't ready to toss in the towel. Trust me, you will know when you are done. Anger has a way of discouraging you and that's when I notice you begin to talk of cashing in your chips. Use that anger to spur you on w/a project or push you to do a new hobby, but try not to allow it to sway your thought when it comes to tossing in your chips. You do not want to make any decisions such as this when you are angry, hurt, disgusted or just plain disgruntled. Major decisions need to be made when you are calm and level headed.
I do understand why you are asking these questions now, i.e., because of his attendance at the wedding. Keep in mind, he may not have attended this wedding if it had taken place last year or even six months ago. However, I do sense from your postings that he's starting to wake up a bit and reconnecting. These are very good signs and huge baby steps. Try not to compare what he's doing now to what he didn't do months, even years ago. Depression has a way of distorting things for those in it. Not making excuses...but depression really messes people up in many ways. As they begin to wake up and/or as the depression fog begins to lift, they begin to take an interest in life, people and events once again. So, I'm not surprised that he's starting to take an interest in people and yes, even attending a wedding of someone who isn't related in any way to him. He could go to this wedding and feel comfortable being there. No one was judging him for what he's done and he could just be himself (whether it's the MLC self or the old H). There were no expectations as to how he should behave/interact w/others.
I think the reason his attendance at the wedding is so important to you is because he's missed so many other special events over the last few years and yet, he can attend a wedding whereby the person isn't even related to him. I know that this hurt your feelings and you feel it's not right, but you have to let it go for now. At some point, you might have the opportunity to talk to him about it...but now isn't the time. Why? Because you are hurt and angry and could very well say something that you can't take back. I am going to suggest that you let it go for now until you are calmer. The door of opportunity will open very soon for this discussion.