I'm afraid I don't have any advice, I just wanted to express my sympathy for your situation. I have to admit I'm not a huge fan of Xmas as I associate it with various historical incidents and emotions, let alone the pressure and commercialism etc. but obviously have always tried to make an effort for the kids. However, this is made 10 times more difficult if you don't know what's happening - because, for example, either the ex and/or the stepchildren won't commit one way or another (or, depending what mood they're in, won't even speak to you - unless it's to demand presents of course) and your plans therefore have to remain up in the air until the very last minute. We have had several Xmases where we've literally been waiting until the 23rd or 24th before someone deigns to tell us what they're doing - and even then, we often don't get the whole picture re: pick up and drop back times etc. It's incredibly frustrating because, apart from being damn rude (I think that's been the objective in some years) like most people we're also trying to juggle the expectations of the wider family and are therefore also unavoidably keeping them dangling too, which I feel terrible about.
Many a time I've tried to say to DP that if we issue an invite months in advance, and don't hear anything back, that as it gets much closer to Xmas we should just say to the ex/stepkids that if we don't hear by such and such (more than reasonable) date, we shall assume they're not coming and look forward to seeing them asap in the New Year. That'd mean we could then be courteous ourselves towards other people who've kindly asked us to theirs and so on, work out budgets for food and travelling, sort time off work and the rest of it. But of course, DP is scared to issue such an ultimatum as apparently it would be seen as "unwelcoming" and therefore we have no "choice" but to sit here like lemons and let other people dictate. (I suppose my "choice" is to go ahead and make my own plans with my kids and family - but that could well provoke a very nasty atmosphere, so I'm kind of stuck really).
Now obviously, every family's different, and if you don't have an extended family to consider too, you may feel more relaxed about the stepkids just turning up whenever if you're not planning on travelling anywhere - but I do appreciate that even if that were the case, you want to know what's happening in advance so you can meal plan, make up beds and so on. It's about being able to get all that needs to be done sorted and "ticked off" in your head so you can properly relax. I TOTALLY get the self employed aspect to this as well. I've had ONE bloody day off all year and like you, Xmas is my only extended period off (wow, six blinking days off a year - and guess who'll be doing all the cooking, planning ?)
Though we no longer have the sort of difficulties you do (but have had in the past) I still have no idea when or if my stepchildren will be making an appearance this year. They've been asked, but are very non-committal right now and whilst I'm prepared to give it a little longer I think I shall have to ask DP to put his foot down a bit more nearer the time as I want to be able to work out what needs to be bought, and, also cement plans to see my own family (which can't easily be done with stepkids in tow due to car capacity, everyone living 100s of miles apart in different directions, ex's refusal to help with travel).
I do appreciate what you're saying about protecting DD if you are in the middle of fractured contact when, basically, you don't know what's happening and what may or may not happen in the future. Obviously, if DD has also been involved in allegations of some sort, this is going to make the situation more difficult. However, even if there were no allegations and it were "simply" a case of one or other stepchild refusing to come to your house for extended periods (we had this in the past - for months at a time, and then, eventually, years - ended up in court. What a farce - long story. Situation manipulated by teen - as in, will see dad for "special" days out, but won't come to house) then it can be extremely damaging to the resident children whose relationship with the "missing" kid(s) is obviously going to be affected if they just suddenly "vanish" one day. My child was a lot younger than yours when we had similar issues, and it was heartbreaking to see them assuming responsibility for the actions of 2 much older kids by asking if they'd done something wrong, or, if their elder siblings "didn't like them any more". However much you'd try to reassure them, they'd still keep asking why, if it wasn't something to do with them, did the older ones not want to see them ? I admit my blood boiled - however much sympathy you have for kids caught up in parental alienation (which was the root cause of our problem) your first instinct as a mother is to protect your own children, and more so, when they're much younger and more vulnerable. Like you, I was also very very wary when it came to the older ones "coming back" after a considerable absence, because I had no way of knowing if they'd turn up, and then disappear again, thus breaking my child's heart all over again. Unfortunately however, you can't insure against that - other than "speaking" to older children and seeking their reassurance it won't happen ...... and in the meantime, the younger child (in my case) just had to take their chances, which, though the objective side of my head knows is unavoidable, the subjective side rages about because I hate the fact my child has been dragged into an upsetting situation they have no hope of properly understanding (heck, as an adult, I won't ever completely understand it). Though at 12 your daughter is older than my child was, I still appreciate your worries - 12 isn't really old enough to understand this sort of thing either .... and they are still more than capable at that age, of feeling very hurt if, for example, an older sibling turns up - at a special time, like Xmas - everything seems "back to normal" and then they go off the radar again having got their presents.
So, as you can see, I don't have any answers, but I wanted to try and explain that I think I know where you're coming from and offer a bit of sympathy. If I had more money, I would have truly been tempted, on many Xmases, to gather up my kids and go abroad for the duration, just to avoid the stress of trying to organise Xmas - when your partner understandably wants to see all his kids - but where the ex is as obstructive and rude as she can possibly be about it all.