I will re-read your thread this evening, but I would say that good boundaries in a WAS situation is that you go ahead as if this is a co-parenting relationship.
You give your H information about things that pertain to the wellbeing of your children, volunteering it when needed, and answering his questions directly if the information is required. If one of the kids is sick, or needs medication, or has a teacher conference, or there's a chance in schedule, you tell him - preferably in writing in a text message.
Otherwise, your children need the emotional safety of being able to speak to you. They can speak to their father about their feelings directly if they want to. You can encourage that if it comes up in conversation. If your H asks you about the kids, you can give him direct factual information, 'Susie had a nightmare last night and slept late this morning so she might not need the afternoon nap,' is fine - he needs to know this stuff and Susie can't tell him herself. 'Susie was crying last night because she doesn't understand why you don't love mummy anymore' may very well be true, but Susie can tell her father that herself if she wants to and feels safe to do so. If he wants to know about their feelings, opinions or thoughts, get him to ask them himself. If he can't have those conversations with them because there's no trust there, that is a consequence of his actions you have no business saving him from.
Married couples parent differently from divorced couples, and this is one of these differences. Let him see what his choice feels like. I know my situation didn't turn around and the out and out contempt from my husband didn't dry up until I started putting this into action, acting as if I was single and he was someone I was prepared to be friendly and civil to but no more than that.
Additionally: I speak more to my H about my feelings around by boundaries now we are in piecing than I did before we reconciled. I think it helps me and my marriage for him to know my stance on these things. When we were separated, I had better results from doing no explaining at all. If he called my lazy when we were separated, I'd have said, 'Ok, bye then,' and either hung up the phone, or closed the door. I didn't really care to have conversations with him when he was spewing, and didn't feel like wasting my time on explaining to him why I had those boundaries. (It took me a LONG time to get to this point - and some great posters on these boards getting very frustrated with the amount of rubbish I tolerated from him!) Even now, you can't explain or cajole someone into respecting you, so if I ever find myself starting to do more than a short, sharp sentence with my H, I cut myself off and leave the room. I don't attempt to control his treatment of me with my words - I just say my truth then get out of there. And thankfully it is a very rare situation.
And hugs. Yes, the contempt is very very hurtful. My H used to look at me with such disgust sometimes. I know now - because he's told me - that he felt he was defending himself against my emotions, my emotional demands, my expectations. That doesn't excuse his behaviour and he knows it, but it helps me to realise that inside that monstrous, angry, spewing man was an immature child who had no idea what to do to get his marriage on track and had no capacity to look at his own part in it so felt helpless to take action.
You aren't helpless though - not now you are here.
