I don't have a lot to add. You're going through an incredibly painful process but with true grace, dignity, self-awareness, and honor. I am just really impressed and inspired by you.
One thing that caught my eye that was especially impressive is that when you noticed he got the date of your marriage wrong, you acknowledge the hurt-- of course that hurts-- but also are able to not take it personally. You are completely right in that it is just another signal of how lost he is right now.
I know it bothers you that your H isn't being a good dad to your younger son. To me, it would feel like I could take anything, but to do it to your child is enraging... but just like every other aspect of this sitch, you can't do anything about your H's behavior. It is all on him. I think you're doing all you can, being there for him. I guess the only other thing I could possibly recommend would be talking to a child psychologist who has experience in D and see if they have any other suggestions about ways to demonstrate your continued love and presence, and red flags to look for in case things are tough.
Hang in there, E. You've got this.
xoxo May
Me (46) H (42) M:14 T:18, D9 & D11 4/19 - 12/19: series of escalating BDs 9/20 - present: R and piecing