I'm so glad social services are involved and have said your baby should be with you.
Can you phone your hv and leave a message telling her that a crisis is in progress and that you are very worried about the welfare of your baby, and that you are scar3d your dh and mil will not return her. Just say that on the message, it's the most important stuff and you can fill in the details when she contacts you - not until Monday, probably, but it's never too late.
I suspect that once mil is kept away from you, you will feel a lot better just for that.
For now, take deep breaths. I mean that literally! In through your nose counting to 3 and out through your mouth counting to five. Do that at least 5 times. Whenever you feel panic/anxiety, concentrate on your breathing and do the deep breathing 5 or more times. It may not make you feel obviously calmer but it's very surprising how effective it is despite that.
Good luck tomorrow. Try not to kick off, no matter what they do. You are working towards showing ss and hv that you are OK. Will someone be with you? Your parents, or your friend? If you have someone there, it may be easier to let them deal directly with your dh, in your presence of course.
I shall be thinking of you in the morning, Hanna.
Do let us know, no matter what happens. We are all rooting for you. Remember, there are a lot of women here who believe in you, who will support you, and advise you, but above all be here for you no matter what.