Swimming, there is no OW right now. Want to know how I know? Because your H is a nut job right now. And you aren't the only person who is noticing. You want to be with him because of the wonderful history you share and your family. Another woman doesn't have that with him, so why bother with a new nutty guy? Why not go see about meeting a new SANE guy?
Yes, I think he is depressed and fighting a battle he cannot win on his own. I don't know how he feels about therapy...could you talk to him about that? My H is completely against it.
Keep up with the no reacting. Keep doing for you and the family and just keep moving forward.
Please please please forget the OW, she's now in the past and you and H have other things you need to concentrate on. She is nothing and never was.