"it feels like he’s been forced into decisions without fully considering my perspective"
He's not forced to make decisions without considering you or consulting you.
He chooses to do that because he thinks you are pliable and if he says I have no choice, you will accept that and accommodate whatever decision he has made.
I think you do have a say in this as he is living in your house (although paying utilities etc - but then he should be) and he has said he will not join you in buying a bigger house to accomdodate his DD.
So your perspective is much lower on his list of priorities. - have you discussed this with him? There's always more than one way to do things, but people will tend to go for the easiest option. Which at the moment is you providing accomodation for them both. A pp further upt the thread said that a 40 min car ride wasn't such a big deal. Maybe these things could be varied so that one week its her having the car ride, the next week staying at yours.
One of the main bugbears seems to be storage and constantly moving the DD's things from one room to another. You mention several times that its you that has to pack things away and get the room ready. If it bothers you so much, shouldn't he be doing this, its his DD? How much of the organisation, entertaining, supervision is he actually doing for her?
One short term solution - until the court is settled is that he could rent a Storage space nearby (and pay for it) to store stuff that would give you more space. Another short term solution to give you a bit of breathing space is that perhaps he could organise an AirBnB for himself and his DD occasionally (particularly if you feel he's not pulling his weight with the DD) when you want to have your DM or guests to stay.
Also very confusing about which mother it was that was passed out on the floor - was it partner's mother who was looking after the DD at the time or the DD's mother. If its his DD mother, the ex, then he's concerned for his DD safety, which is understandable. You say he's going to court over custody... are you prepared for him winning a high level of custody. It doesn't sound like you would be happy with that.
Otherwise, and outside of his needs and his DD's needs - it does seem that you are really fed up of having set up and pay for your own space, and now having to share it, or have someone else deciding how it is to be used.
I think you are also fed up of being at the mercy of what his ex wants and doesn't want, and see it as someone you have not connection with imposing her decisions on you whilst you have no say in it. I'm afraid that again is down to your partner. It comes across that you feel you gave them an inch and they are taking a mile. Again that is your partner, not his DD and not his ex. I feel sorry for the DD caught in the middle of this. Its not her fault.
If you find it difficult to get your point of view across to him and for him to set boundaries with his ex... for whatever reason.. perhaps you and he could go to counselling together to hash something out and see if there are concessions to be made on either side.
Maybe you aren't cut out for living together and he needs to rent his own apartment for a while whilst you both sort out how you want to proceed. But then he may not agree to continue.