If S3 is often asking you why you are sad, then you may need to try harder to put on your happy face around him. If he sees you smiling and uplifted when Mommy comes to take him home, yet you are sad when he's around, he could misunderstand and think he is the problem. It is really tough to put on a happy face for our kids when our heart is breaking, but we can be our strongest and best selfs for them.
This may sound anti-DBing to your ears, but I think you need to stop focusing on how to fix the situation with your W. Stop thinking about what might be troubling her, or if she looks sad, etc. Turn your attention on self healing, rather than relationship healing. Even if she came back today, you would mess it up b/c you have not healed enough to deal with it. You are still reacting out of your pain. You are in no condition to fix the relationship, until you have time to heal yourself. Do you understand what I mean? You are putting all your thoughts, time, energy, feelings, and attention into her. When you stop doing that, and you start putting everything into your own healing and growth........then your life is going to shift from this pit you are in currently.
As a former WW, I can tell you that everything you say to her is emotional pressure. Just your physical presence is emotional pressure on her. Not in the way you might hope.......like making her feel guilty for what she's done......and even then, it makes her want to get away from you as fast as possible, or avoid you altogether. Asking her about her weekend is not appropriate. Making references to her facial expressions and asking if she wants to talk is not appropriate at this time. I realize these things are difficult to accept, but they work against you......not for you. ((hugs))
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!