Hmm,
I agree with the others that you need to find a different house. Or a different way of doing things. Something that works for the whole family. Or don't get married.
From your posts, I hear a lot about how it's your house, you've got more equity, your son's needs etc... but I do think you need to be thinking much more as a couple. Isn't marriage supposed to be a Partnership. "What's mine is yours" and all that. If you're going into the marriage with this attitude, then I do wonder whether you need to consider whether marriage is right at all.
Whilst you've said you're putting more equity into the deposit, how much is your dh paying on the mortgage? Are you moving from a large two bed to a three bed? How much will you and he be paying towards the mortgage? If you earn more than him and will be paying more than him, then maybe you've got a point, but If he earns more than you, and will be paying more than you, then your argument that you paid more deposit is somewhat negated.
I also get the vibe that perhaps you don't like your dsd much. You said things like she says she has got anxiety. Well has she or hasn't she? Because if she has, then I think you're being pretty mean casting doubt on that. It can be crippling and should be taken into consideration. It sounds like she's still at school / college, with the prospect of being thrown out of her home at 18... mum going abroad, that's a lot for a teenager to have to cope with whilst doing her A Levels. And from her perspective, losing her home, her room, her security to move into a tiny box room, whilst the big bedroom sits empty half the week is harsh to take. I really think you need to try and see it from her perspective too.
Clearly this isn't the house that's going to work for you. But could you look at alternatives? If you can't afford a four bed, can you rejig the walls upstairs to make the rooms a more even size? Can you find a cheaper house and get your DP to do an attic or garage conversion to create that bigger bedroom?
My daughter's bedroom is in the garage conversion, to avoid her having the box room. Once she moves out, we'll turn it into a study.
You're becoming a new family and I really do think you need to find a solution that works for you all as a family. Going ahead with this house sounds like it will only cause anger and resentment from whichever side loses out.