Nicole, I think you nailed it with the guess of him feeling guilty. As soon as I read what your D asked him, and then his actions at her birthday party I think it triggered guilt. My guess is that he would love to say "stay, don't go, we'll live in the house and work on R", but that he feels that it is too late? I hesitate to say that since I don't want you to act on it, but it does seem like he is having a war inside of himself.
My wife was the most depressed and the saddest after BD when she started having second thoughts about what she was doing. I could see the war going on inside of her and the outward showing was depression and sadness. Once she came through it she was much more invested in the MR again, though a piece of her still wasn't ready to give up the fantasy.
10 and half years later we are still heartbroken about Larry. I truly believe he would still be here today if Liza hadn't already put stress on to his already fragile health.
And your D's question to your husband breaks my heart. Don't these WASs realize or care what they are doing to the kids? It really struck me when MWD talked about being an adult and still being sad that her parents marriage ended. It ALWAYS has an affect on the kids no matter what people want to think or believe.
M(53), W(54),D(19) M-23, T-25 Bomb Drop - Dec.23, 2017 Ring and Piecing since March 2018