Wow, that's a couple of crazy days. Your W is clearly going through something---sounds like an identity crisis: who is she? what does it mean to be a mom? can she integrate the life she wants with motherhood and M? That's a tough thing.
Maybe it would help if you sit down and ask yourself: Do you want her back? I know you don't like the things she's doing right now, but if you really want her back, then you have to let her walk her path, find her own way.
A couple thoughts that popped out at me: You feel like you're enabling her, staying with your S while she parties. If you decide you really do want her back, you have to drop your expectations that she'll be the mom you'd like her to be and the woman you want her to be right now. Stay with your S, make it about him and your R with him. If you don't have any expectations about your R with W, this should be (relatively) easy.
The other thing: Kissing, being asked to spend the night---these are keeping your expectations up. If you're willing to let her go through whatever it is she's going through and still be there for her, perhaps it would help if you stopped the smooching. I know you love her and that stuff feels great, but I'm thinking it's keeping your hopes up that she's coming around, keeping your expectations up. If you can't kiss her, flirt with her, etc, without feeling like you're enabling her, then stop it. Be her friend, be a great father, and let her go.
With the expectations gone, every little move on her part is something you can observe with some distance but not react to, not start hoping she's turning around. Also, you don't have to feel like she's using you because you'll know why you're doing what you're doing. Does that make sense?
You don't have to feel used if you have clarity about why you're doing things. And if she asks, hey, can you spend the night, you can say you have plans, or ask what she has in mind. You're letting her control how you feel right now, and that place of powerlessness just engenders resentment. Figure out what *you* want, then walk with integrity.
