FWIW I think too you have been very brave in admitting your fears.
I'm sure you have sympathy for DSS but it could quite possibly damage him even more if he came to live with you and you found, that despite your best efforts, for I am sure you are not a monster, you literally couldn't give him the emotional commitment he needs. It's one thing knowing in theory what needs to be done, but another actually doing it when it comes to love ..... you can't switch it on or off, and I think it's very hard to pretend you care for someone in that way when you don't - instinct will nearly always mean you'll be sussed.
I do wonder however if you'd feel differently about this if your DP was more supportive. From what you've written it sounds as if your role as chief child carer is taken for granted already, and it's disgusting that he slopes off when his son comes to stay as surely, the main point of him coming at all should be for him to spend time with his father. Similarly, it's one thing to have a full and frank discussion about the inevitable changes which'd happen were DSS come to stay permanently, but quite another for him to have already worked out your role in how the practicalities are going to be managed. It strikes me that his life will change very little except he'd obviously see his son more after work etc., yet on a purely logistical level you are being expected to take on a lot more work and responsibility. This would be scary even if DSS didn't already appear to be disturbed.
You know ..... so often on these threads, a step mother dares to complain about something step related and brings forth a torrent of criticism upon her. Yet time and again, as the story unfolds, it becomes clear that the dad in the scenario isn't pulling his weight in regard to his own children and/or taking his wife's hard work for granted. Additionally, while the OP is being berated for voicing her doubts the fact that DSS is apparently favoured by DP when they are all together hasn't received any response. Look ....... we are all mums, of course we are going to feel protective of our children first and foremost and if we are only capable of giving so much (emotionally), what we give is going to be given to our kids - no matter how much compassion we feel for another child. We are all different and some people simply lack the emotional reserves to take any more responsibility on, or, are unable to give what's required because the person in question is, quite literally, too much of a challenge - or, dare I say it, actually unlikeable - and yes, even children can be unlikeable in some circumstances - they are, after all, individuals, just as adults are, and no-one expects you to like every adult you meet.
Now, despite having said all that ..... I do agree that when you get together with someone who has kids, there's always going to be the possibility of those kids living with you one day but be fair, the shock of that becoming a reality is rather different to knowing, without any doubt, from day one, that your partner's kids will be sharing your home. The OP's partner knew, for definite, that if he wanted to be with her, that her daughter came too - and he accepted that. The OP didn't know, for definite that one day the boot might be on the other foot so to speak and I suspect she probably gambled on the fact that it was unlikely to happen .... which, as it turns out, was the wrong thing to do, but it's a pretty human reaction to such a situation. No matter what she "should" have thought or "should" have been prepared for, I don't think she's a monster and I appreciate she's had a big shock.
Another thing to bear in mind is that things change. 2 years ago she may not have realised (because there hadn't yet been enough evidence of it) that her DP wasn't a particularly hands on dad, and that he would nearly always take his son's side in arguments, regardless of the ins and outs. 2 years ago, the idea of DSS living with them may have been something she believed she could potentially cope with, but now, having seen her partner more or less expects her to do it ALL, the idea seems much more frightening.
The ONLY way something like this is going to work is if both adults always present a united front and sort out any disgareements about discipline away from the children. There has to be a clear set of house rules and boundaries so everyone knows where they stand and absolutely no favouritism. Both adults also have to be totally committed to pulling their weight .... no way do I agree that it's MP's responsibility to raise someone else's child, even if he is her partner's child. That is her partner's duty, albeit with her support. Additionally, it's also likely that DSS may need extra support of his own re: his behaviour - or at least, some sort of assessment in the 1st instance by a trained professional to evaluate how serious (or not) his problems are. Again, damn right MP should be concerned about the possible/potential impact on her daughter - what sort of mother would she be if she ignored a potential threat to her child's well being ? The issue for her DD is the impact itself, not who perpetrates it ... it doesn't "matter" if it's DSS or the kid from down the street, it's the end result which is of concern.
Everyone who's demanding she "should" do this, she "should" be the adult etc etc etc, consider this: despite having serious misgivings and doubting her ability to come, she nonetheless agrees to having DSS and some time after he's move in, her fears are realised because she can NOT cope. What sort of effect is that going to have on ALL the children in the family ? ..... because the unhappy/hostile/argumentative/stressful atmosphere is bound to be picked up upon. How would she then feel - say, in 2 or 3 years time, if she had subjected her children to a situation she had instinctively feared would be an unhappy one ?
By being brave enough to discuss these issues now, MP is maybe avoiding future heartache not just for herself, but also for her partner and all the kids (though I admit there would be shortterm heartache if they split). I think it would be wrong for her to go into this if she wasn't 100% committed and that, is what I get the impression, she is trying to get her head round. At no point have I formed the impression that she, MP, is suggesting DSS goes into care while she remains in a happy family that includes his dad but excludes him. It was DSS's irresponsible mother who was charming enough to suggest that he went into care !
MP - despite what I've rambled on about, and I hope it makes sense, I do feel, in an ideal world that you'd all be together as one big happy family and you & your DP would be able to work together to turn DSS round. But that's in an ideal world and I fully appreciate that we all have different limits regarding what we can cope with. I think you're in an incredibly difficult situation because you are facing one of the most difficult dilemmas possible: take DSS in, despite fearing you're not up to the job (and having understandable doubts about whether your DP is too !!) and worrying about possible adverse effects on your children, or, split up, leaving DP & DSS together, but at the same time, disrupting your daughter and taking DS away from his dad.
All I can say is that you have to be true to yourself regardless of other people's ideas of what you "should" be doing. They aren't in your shoes. I really hope it doesn't come to you & DP splitting up, but I tend to think the key to all this is how your DP is going to commit to supporting the huge change in your family dynamics, and whether you can trust him to step up to the mark. If he can't do that I imagine you would very quickly be filled with resentment, which won't do anyone any good - including DSS.
I really feel for you because I guess time isn't on your side and this decision is going to have to be made very soon. Again, in an ideal world, there'd be plenty of time for detailed discussion and perhaps couples counselling too so DP understands it's unfair for him to leave you doing all the child care, as well as you (both) coming to a fair and workable way of disciplining all the children, no matter who they "belong" to.