While I cannot help you much with the custody thing (I have full), the thought that you discussed it being an issue for S is positive. It is always the WAS that maintains "the children will be fine, adjust etc." Perhaps a longer stay between households?
My children are older and S16 knows of Hs GF, but I refused to allow D13 to either know or meet her. It has been 7 going on 8 months though and since doofus H has changed his FB to in a relationship with...I think the time has come for H to tell D. Not looking forward to that fall out. I have also said that D and H time is just that D and H time, not include GF time. He sees my kids very little though, since we live about 60 miles apart.
Does your W live with OM?
Perhaps you can ask her to limit the interaction with OM and S until he settles a bit more. This is not unreasonable.
I am betting she knows what solution you want, but at this point, it is not what she wants or needs. You can only control you at the end of the day and your responses to situations and interactions.
