Great!! 14 weeks sober. I know you are in the UK, and probably don't drink it, but here in Alabama, even though we like our beer or wine, we love a big glass of iced tea with a nice juicy steak! With lemon.
I think your W is certainly feeling the stress. You said she was under stress when y'all were together? Was she always stressed out then? I would say you are right about S's behavior. Sounds like she is getting a taste of what it is like to have the kids without a H in the house to send them to when she gets fed up. I don't imagine it will get any easier anytime soon. And, like you said, the om coming around will make her more stressed out getting things ready for him. That will get old, soon enough.
It sounds like your R with the kiddos is getting better all the time. This sort of situation is hard at best for the kids, but having a well-grounded parent in their court is going to help them get through it. They both know you love them and will take care of them always. You are a good father.