You're an adult, if you really can't bear that your mother has gone back to the religion that ruined your childhood then you have to make decisions now for your own well-being. Your mother was a widow and started a relationship with a married man that turned into a very bad marriage. The divorce damaged her and now she's blaming a wrong choice on leaving the religion.
I don't have a relationship with my siblings, my JW mother died years ago. After a while I couldn't stomach their silly passive-aggressive games to bring me back. I realised that the so-called acts of love, shunning, we're just punishment for hurting them by shaking their faith when I left. I just get in touch if I hear they are seriously ill, then watch them drop me again, which I can handle.
So it's up to you, make your decision for your own health and happiness. I assume when you talk about coming out you mean telling your mother you're an atheist. If you decide you do want to keep some sort of a relationship with her, why tell her what your beliefs are now? People don't normally talk about beliefs or lack of them outside the JWs. It's up to you but if it's going to shatter your relationship and you do want to keep it to an extent, why mention it?