I have a similar issue although the means of control are different. What I am trying, and I have only just started to try this, is to stop trying to fix the situation. By which I mean, instead of telling yourself his behaviour is awful, controlling and manipulative, and you should close it down more and not let him get to you, acknowledge you have done everything you can and that he is a controlling person and it is okay to feel awful after seeing him. Add to this positive self talk that you are coping really, really well with a difficult situation, that you have negotiated SS and police involvement (and these organisations do not necessarily help even if people assume they do), and that DC feel safe with you. The interests of DC override your own interests, so if DC want to come home, then they need to be able to do this. This means you have to cope with uncertainty and with meeting him. It is good that they can say they can come home.
Rather than fighting his behaviour, maybe be gentle on yourself and accept you have done all you can to make things manageable and focus on the time you do have to yourself and DC. What are the things you do after contact to recover? How many times does anyone tell you, or do you tell yourself, that you are doing well? Rather than it should have stopped by now. It won’t stop, because otherwise it would never have started.
You cannot make it stop and it is setting yourself an impossible task to try. The police, SS and the courts have not made it stop, and you have got heroically far by the sounds of it. Be proud of yourself for that.
So the question is what do you do for yourself now, given that you cannot fix this situation any further? Not how can you fix this situation further.
Does that make sense? You cannot fix a problem which you have not created and which the law perpetuates (because contact is ordered). You can only fix how you respond and minimise the effect. Get counselling for the trauma if you have not already done it. Work out what home-coming rituals you can have for DC to mark the transition from weekend to home. If he is routinely dropping them on Sunday, can you go back to court and ask for a Sunday drop off? If you face that head on, that he wants to unsettle you with the uncertainty of Sunday drop offs, then you reduce its power. Maybe he would then shift the uncertainty to earlier in the weekend - who knows? So maybe do nothing and focus on the things you do know.
I am wondering if you gain power by accepting that his shit behaviour could happen and you could cope with it, and doing other things to make you stronger - meditate, exercise, see friends, all of those things which are bigger and more important parts of your life than him.