I am so very sorry to read about your situation. This is a shocking and tragic development for you and your family. I totally understand what you mean by living in an alternate universe - for me, it felt like my H was possessed by an alien!
The situation with your children and custody sharing and presence of OM is a legal issue. Do you think you had the best attorney possible and that the results were typical for the area you live in? The law and practices are so different from place to place. Only a local, experienced attorney can tell you if there is anything you can do about it, and if OM's behavior pattern (multiple affairs with married women) would be a factor that could keep him away from your children. I also have to wonder how long it will be before he cheats on her...
On FB - if I were in your shoes, I would not approve the FB-request, because it could tempt me to keep checking what H was up to, and that would only cause me pain. We have to try to focus on our lives and try to find pockets of happiness and distraction (I know that can seem difficult at this point).
Are you seeing a counselor for yourself? That can be very helpful. For me, part of the help with counseling is also that I can use that time to focus on the grief and pain and questions I have, so I can function the rest of the week. Journaling helps me sleep at night when my mind is full of confusion and pain - I write it all down so I can put it aside and sleep.
You probably won't believe what I have experienced - that right now, after H's affair, regardless if we make it as a couple or not, I feel like I'm in a better place than I have been for almost a decade. I have been given a chance to develop as a person, look deeply into my relationships, and hopefully improve my marriage and make it better than it ever was. Even if my marriage doesn't survive, I will know so much more about myself and what I need to do in a relationship than I did. Did I think 3 months ago that I might look at H's affair as a blessing in disguise? No way!
M 16 yrs, WH62, P54 3 adult blended kids EA 11/13, BD1 6/14 PA fall 14, BD2 2/15 Piecing 2015, BD3 12/15 Separated 4/16 WH moved OW in 5/16 Divorced 6/15/17