Another thing is that your XW has never 'harmed' your child, from what you've told us, she is not mentally ill or massively unstable, she does not hit him all the time or stub cigarettes out on him.
All she is doing is dating, which is hurtful, and I know it makes you furious and taken for granted and sexually rejected and jealous etc because I go through all those things thinking about my X dating other women, but the fact is, that is separate from S5. You CANNOT stop her entertaining people in her home. No matter how hurt you are, it's a free world.
You knew that being D, this was a possibility. If you never get back together, you knew that chances were she would look elsewhere, as most people do. This has nothing to do with her mothering or S5, and she is not going to be doing anything sexual in front of S5 anyway, so you cannot say it is a child protection issue or that he needs 'protecting' from her.
I think the issue here is your hurt and betrayal and feelings of anger and loss and jealousy - which are entirely valid and real, but still have nothing to do with visitation or custody.
If you were happy with S5 residing with XW all this time, which you have been separated many months now, why would that suddenly change because she is dating?
From your posts, I cannot see that she has done anything wrong with S5 at least.
Please think carefully about your motives, and don't let hurt and revenge become part of your decision making process.
Sorry if this is a bit blunt but I'm not the kind of friend that would go 'there, there' to whatever happens.
