I was thinking of your parenting schedule today. And a thought occurred to me. Now, I may get flamed for this but I will be a little crazy tonite and put it out there anyway.
The one thing we know for 100% certain about WH is that he is absolutely crazy about his S. Which is great. We also can probably assume that it frustrates OW that he wants to spend so much time over at your place - no doubt because of her own insecurity and loss of control over WH. This is of her doing for deciding to be a homewrecker, and she also needs to accept the consequences and it sounds like she doesn't want to.
If it were me, I would delay the overnights/long Saturdays for as long as possible. WH can come over to your place to visit S as often as he likes. Why? Because I think it will really create an issue for WH and OW. I personally think that she is probably pressuring him about this, and putting WH in a bad spot. Eventually, this should blow up between them if he is giving her promises of things like more time at her place with S and then it doesn't happen.
I am sure even if he is as happy as a clam with OW, while he is there with her his mind probably continually wanders over to how S (and you) are doing. OW probably senses this (And may even get annoyed by it). Allowing S to go over there often may result in less of that and normalize the situation which isn't good for you.
