Actually Daisy on reading the info provided in this thread, many people would think it "quite awful" that within 20 minutes of the OP's DP "getting the call" he'd set off to fetch his daughter. I'm gobsmacked that he did that without much of, (or any) conversation with his pregnant partner regarding such a momentous change to their household. Even with the most co-operative and willing stepparent who's completely open minded about such a proposition I simply cannot believe that just 20 minutes would have been sufficient time to properly discuss the impact, the rules, the boundaries, the practicalities, the logistics, the finances, the feelings of all concerned etc etc etc before she was brought back to the house .... and only a naiive fool would think that was a good start to a new chapter in the whole family's life.
This is not as simple as a "tit for tat" transaction where, because, one party willingly lives with the other's children, all things become equal. The DP here presumably chose to move into OP's house of his own accord and without a metaphorical gun held at his head. I expect he did so not as some act of charity ("expected to live with your children" indeed) but because he wanted to move his relationship with OP on a step and felt confident he could live contentedly with her children. If he had any misgivings about her children I don't suppose he'd have moved in at all.
On the other hand, the OP has explained that his child made serious accusations against her - who'd feel happy at the prospect of sharing their home full time with such a child ? Furthermore, she's stated that although the truth eventually came to light she was made a "scapegoat" in this issue which implies that the child was never appropriately counselled and/or "punished" (as in having to face consequences for her actions). This would make all but a stone-hearted robot fearful about living with that child - you'd feel frightened in your own home in case similar lies arose again .... and that is especially pertinent in this case given that OP has had sole care of this child dumped upon her because her DP works away for extended periods, meaning that OP would be particularly vulnerable to accusations if she's with the SD on her own most of the time.
NO WAY - given the background - should OP have had this presented to her as a fait accompli. How dare you call her "selfish" Daisy ?!?!! ........ when her partner can't or won't address the very serious issue of false accusations, and whilst she doesn't expect him to do any practical or financial parenting for her children. Yet she is "expected" to parent his child, without any say in the matter, without any previous discussion and laying down of rigid boundaries (which would be absolutely essential in this case) !! There's only one "selfish" party in this relationship and it's not the OP.
Ginger ... I'm sorry you're now having to consider becoming a single parent again but can completely understand why you feel so unhappy and although it'll be hard, I'd prefer to be on my own than to effectively be forced to adopt a role I haven't been consulted about and where I couldn't trust my DP to co-parent in a consistent and fair manner. As you say his lack of enquiry about you and the baby must be very hurtful indeed - it's quite unbelievable.
And ignore Daisy .... if I'm thinking of the right person, this particular poster revels in shit stirring should anyone dare confess to being a stepparent who isn't in total thrall to their stepkid(s) regardless of what those kids are actually like.