I know what it can feel like to be so close to a situation that it becomes hard to see the big picture, but I have to say that from "out here" it looks as though your H has made remarkable movement forward over the past 1 1/2 - 2 weeks. I have worked in a clinical setting for the past 20 years and I can tell you from that experience that mental health issues do not change overnight. In fact, if someone tells us that they HAVE changed overnight we are concerned that their change is tenuous at best. Now I know that to us out here it seems that many of the MLCers who come to the LBS and take responsibility for problems in the M have had sudden epiphanies, but for those who are in the middle of these situations the changes are really painstakingly slow.
Given what little we know of H's life, it appears that whatever has transpired in his life in conjunction with your going dim over the past 2 months have served to move him forward. I'm actually very impressed that H has consistently been receptive to you in spite of some emotional "back-and-forths" during this time. He hasn't scurried far back into the tunnel with these little "dust-ups". He has stayed just outside the tunnel it seems to me. He has made the HUGE step of admitting his role in what transpired and talked about reconnecting with your parents. Based on what you know, it appears that he is spending VERY little time with OW, much more time with you and D, and has begun to do caring things for you.
Dealing with OW is another big step on the continuum forward. If you don't do anything to scare him away, H may very well take this next big step forward in the next few weeks. Jody often says to me that I can consider the things I am doing to DB (i.e. learning to be more flirtatious, being more giving, etc.) to be practice for myself for my future R with XH or someone else. I think that right now you can look at learning to deal with this uncertainty and the anxiety it evokes as an opportunity to practice how the new-and-improved Cas will handle anxiety in future R's, with H or someone else. You can always back away a bit without losing all of the progress you've made if you are feeling spent or taken advantage of. Now that you and H are communicating honestly you might even tell him in a kind way "H, I am feeling a bit (fill in the blank here) right now. It would really help me feel more (fill in the blank) if you could do (fill in the blank; give him something specific and do-able at this stage so that he KNOWS what will help you but don't expect him to be able to do this right away --- remember what Rabbit posted on my thread awhile back about Mr. Rabbit providing her reassurance?).
From what you've told us it sounds as though H is definitely moving OW out of his life, but this will be a significant loss to him. It may take him a little time to do this. That's why there's a MLC stage called "Withdrawal", during which the MLCer often mourns the loss of the OP. I think there are ways to remain in H's life while not compromising your integrity during this process. You have come incredibly far on this journey. Be patient Cas and you may be able to complete the journey you initially sought to make. Baby steps......baby steps......
