Coly, I think people have different opinions on the role/responsibilities of step-parents, and especially after seperation. Your D is also old enough to have an R with him outside of your M. So it's not so simple to say what you should do to facilitate that or what he should do. I am of the position that if you marry someone that has children, then you assume a role as a parent to that child if the M survives or not.
Your H was the adult when he met your D and he remains the only adult in his R with her now. I think it is up to HIM to keep that R going with her. I don't think it's fair to you that you have to worry about their R so much. I also don't like that H wants freedom and feels more entitled to it because she is not his bio child. That's crap and incredibly selfish. He marrried a woman with a child and he cannot simply unchose that role and modify it (if this is the case).
However you cannot control him nor I hope that you want to. I think it's not attractive and I would question how much you want to be with a man that can be so selfish. Is he having a MLC and not thinking clearly? Is he deeply depressed and in crisis? If these things are true, does that make his beaviors and neglect more forgivable? I think only you can answer that. I am still not sure he is having a MLC (I don't see it in your posts) or if he just stepped out of his M, having an A, or just wants his freedom and cake too.
I just don't want to see anyone use the term MLC as an excuse or rather explanation for hurtful behavior. It is still the actions of another person and they are still responsible for them. I may be too harsh, but this is how I feel.
My H is not the bio dad of my oldest child (now an adult). His actions hurt her tremendously and her being a troubled teen was deeply affected by what she saw happening to me. Long story. He damaged his R with her and it was something that she has to learn to forgive and it was another strike against him that I have had to see past.
All things to consider as we choose a partner in life. My H has had to do some major work to proove that he is not only committed to the M but to oldest D as well.
Blu
“Forgiveness liberates the soul. It removes fear. That is why it is such a powerful weapon.” – Nelson Mandela
