There's a few things going on here and I think some things are reasonable and some things aren't. But to say you are being totally unreasonable is, I think, very unfair.
Firstly, your partner should be helping you with the night feeds. End of. People saying "he has three kids and needs to consider all their needs" are not being fair about the fact that a five year old should not need their dad all night, a baby will do. I don't think cosleeping is child abuse but I DO think children need to learn to sleep on their own by this age, and it's an incredibly silly and unrealistic choice when you are separated and your child moves between two houses. This is the mums fault, and a lot of mums who do this would undoubtedly turn around and say they are not comfortable with their child sharing a bed with the step parent so they know full well they are causing you a problem here, because they are sending their child with unnecessary sleep requirements into a household that can't naturally accommodate them, because your partner shares with you and in this case is also responsible for a baby.
It's very unlikely that you would be able to improve her problems with sleeping without the mum also working on it, if that is where she spends most of her time. I would reason with the mum that choosing not to teach her daughter how to sleep on her own is leading to her having to be upset every time she visits her other parent, because it's not possible at yours. A vast majority of children her age have learned to sleep by themselves and I do think that choosing not to teach them this knowing full well you are separated from their dad is often either deliberately obstinate or cruel, because you know their expectations can't be met elsewhere and set them up to fail anyway.
That said, it's very likely the ex will dig in her heels and refuse to adapt, in which case I would just stop your partner sleeping in there with her. It's a shame as she may be upset, but there's nothing you can do about the fact that at five she has no physical need for someone to be there with her and she simply needs to grow out of it. Your partner has a responsibility to do his share with his baby.
The bedroom is a bit more of a complicated issue, because I'm not sure why you decorated her bedroom as if it was going to be hers, knowing that in a years time you would need a room for the baby. Her sharing with your other daughter would not have been unreasonable, but it's more unreasonable to promise her a room and then take it away from her. It's still a bit early for your son to need his own room yet but obviously he eventually will and he will be living there full time, so it would be ridiculous for him not to have one while hers is empty most of the time.
We have recently moved to a bigger property so my baby daughter and 5yo SS can each have their own room, but before we moved we had my daughter in a travel cot and would move her into SSs room during the week when he wasn't there, and then back into ours for his visits, so it was only two nights out of every 14 that we were disturbing her sleep by being there. If I were you I would go with this solution for a good few months, whilst at the same time weaning your step daughter off cosleeping to get her to a better place sleep wise. Hopefully, when the time comes where your son simply needs that room, she will be a good enough sleeper where she and your older daughter view it as fun to share. You will have to hype it up like that, try and make her excited to move rooms.
If not, then sadly you'll have little choice but to move, but I do think a bit of tough love may be required to avoid this.