I understand how stressed you must feel, especially that powerless feeling as she is an adult and you could not forbid her to go back to him, yet you want to protect her and worry about her actual safety. So I think it could be helpful to look at the situation on a practical level first. The baby is due in February so you only have to survive at the most 8 weeks of this situation. You will want to give the best support for your daughter and at the end of this period , so although it is very hard , you need to find a way of dealing both with your feelings about her partner and actually speaking to him and not letting your body language say something else!! So do you have a very close friend , who you can confide in and let off steam about the situation? That way , you might have to grit your teeth when you are with the couple, but can go home and have a rant to your friend. Have you got an old pillow or cushion that you can clench your fists and bash into it thinking it should be him you are doing it to.!! For your daughter at this time, you have to think what you are going to say , as it is easy to say the wrong thing and then in the future have it thrown back at you. So, you might say something on the lines of saying that it can be exhausting the first few weeks and if she want a nights rest or whatever, you could offer to go over and be the one to get up to the baby, if she is not feeding it herself and she can stay in bed and get a bit more rest. He doesnt sound as though he will be likely to get up anyway, so he might actually see you as being helpful, rather than protecting your daughter and her child. What you might do now, is look around for a decent b and b close to where they live. Then you would be able to go to help out if it was needed but at the same time, have somewhere , where you will be looked after and to be able to go back there to get a break and calm down. Of course on you way back there drive to a more remote area away from people and then get out of the car, scream, swear at him and let some of the tension go into that .
You have already said that you are adding an extra bedroom into your hunt for the house. So although it may not have been in your plan, it will be very good that you will get a house with that possibility of an extra room, and you might also look at the house to see if it has the possibility to have a sort of bedsitting room, so that she will have her own space, within your home. That way although she will be under your roof, she will still feel like an adult, and be responsible for what she does and what she can decide for the future. The slight worry is that , naturally just having had a baby, her hormones can add to the difficulty of adjusting, and also when our children come back to our home, it is easy for them to slip into the child role again. However as it will be a new house to you all , that should not be a major problem, but making sure that at least in her room there are enough plugs etc so that you can put say a kettle, microwave or air fryer there. So in an unspoken way you show that you respect her as an adult and expect her to want to do things her way and not revert to being a child. On the plus side, if you need to do this, your reward is that you will have been close to her and the new baby in these first vital weeks. You will have a bond with them both that will be special to you in the years to come, whether they resolve their differences or split up.
Lastly I would suggest that if some of this comes about, that you and your husband try to spend some time together , where you are not just thinking about them, but going out for a coffee or a meal together, perhaps going swimming or for a walk and of course you have your other daughter to care for too.
I have moved 19 times as an adult here and abroad, so I just pass on an idea that we used. Once you have found the property and are going through the buying process, get some graph paper and draw up a to scale drawing of the house. Then on the same scale measure your main pieces of furniture and cut out the shape. Put a number on the piece, and have a piece of paper with the items number and description. Then you can put these little bits of paper on the plan and work out what looks best in what room and how you might change it, or try a different idea. It is fiddly to do and you really need to check that you have the exact size and shape of the furniture. So , we all have our favourite pieces etc. I am a singer and had a large upright piano, quite old but a lovely tone. I wanted to take it with me so much and messed about with the plan trying every which way, until I was able to admit - which I had known already really- that there was no way it was going to fit. So regretfully I sold it. But you see if you do that it is a lot easier to have all the things and put them together like a jigsaw on paper and not feeling shattered moving the actual furniture round when you get there., oh and dont forget to note which way doors open. It gives you a chance to think how you might like the new place to look. You discover what will just not fit, and get rid of that. So you are getting rid of things rather than taking them with you and saving on the costs. You also have quite a good idea of where you want things to be placed. Cuts down on stress and costs!
I do hope these are ideas that might be of help to you, Of course if there is no close friend around we are all here for you and if you cant sleep at 2 am ,come on here and have a rant and moan and you will feel better for it anyway, and someone may have been in a similar situation and can offer a helpful word. It doesnt matter how old our children are, they are always our children. My son is 50 this year, but has had an operation on his ear some time ago and now he is having recurring problems. They have now found a hole within his ear and he had to have an MRI scan on monday and it is going to need a major operation . I have been on tenterhooks to know what is happening . He had said he would ring me about lunchtime and didnt ring until about 8pm. I was desperate to know what was happening but knew he would not want me ringing him. He needed time to deal with whatever he was told first. So I didnt ring but I felt very tense and couldnt even go out for a coffee or anything until I heard from him. Of course I didnt tell him that ,and I know he is intelligent and has his wife there too but it is just that feeling you will always have for your child. I hope all goes well and you will have the joy of your new grandchild, so your year will have stresses but look forward to that special moment. I am listening to music from Vienna, and remembering many varied new years eves in the past. Now I am a widow and live alone, but I shall go to my door for midnight, raise a small glass of whisky and welcome the New Year in, and wish all my family , friends and those who are ill or stressed out , A Peaceful and Healthy New Year. You will be part of my thoughts there too and I hope that 2025 brings happiness and a better year than 2024.