Yes OP, I think you have had something of a hard time on here. It's true that many/most parents don't get "time off" regularly .... I certainly don't now DP and I have our own child, but that's not to say that most parents wouldn't appreciate some "me" time every so often and if contact weekends are dictated by inflexible exes then there's nothing wrong in taking advantage of that.
Having said that, you obviously have nothing to lose by at least trying to get one or both of the exes to be a bit more accommodating. You wouldn't, for example, be asking your ex to have his girls more often - but simply at different times. I appreciate it might not be possible with whatever his job is but if, for example, he were to have them 2 weekends in a row, while SS still came EOW, that'd mean that you'd have 1 weekend with ALL the kids, 1 weekend with your girls and 1 weekend just adults (I think). The same setup could obviously also apply if SS's mum agreed to send SS over 2 weekends in a row - but again, appreciate that arrangements can become terribly complicated when people are a) rigid, and b) in her case there are 2 of her exes to negotiate with.
Anyway, the point is, that if you could possibly arrange things as above it'd mean that the conflict and practical difficulties you describe would at least be limited to just one weekend a month instead of two. E.g. SS could sleep in the girls' room if they weren't there. I understand that he isn't well behaved and, apparently, damages stuff (deliberately ?). If so, then you need to have a very serious talk with your DH and stress that if you're to find a way through a difficult situation that isn't ideal for anyone then it's absolutely vital that you both need to apply a consistent approach to all the kids (age appropriate obviously). It's ridiculous that DH won't discipline him - there have got to be house rules which all the kids understand, and consequences when they don't obey. I'm sure your feelings of stress, frustration and being trapped are exacerbated by seeing your DH do nothing to curb bad behaviour. Obviously, deliberate damage (if it is) is completely unacceptable and SS is old enough to be punished for that.
Similarly, in such a confined space it's bloody ridiculous to have an Xbox on all day. I'd be seething at the monopoly of the TV, and the annoyance factor, even if it were inoffensive games but what the hell is your DH thinking of by letting him play him play COD - which I'm sure are all either 16 or 18 certs - at the age of 11 ?! If they're DH's games then they should be put away where SS can't get to them - but regardless, even if they were on open display on a shelf a firm "no, you're not old enough" should be the end of it. It's totally irresponsible to let him play - and appalling that your much younger kids should also be exposed to this. At the very least, playing violent/scary games isn't going to put SS in a calm and tolerant frame of mind is it ? I really don't know what else to say ..... I'm just agog that your DH allows this, and must think it's okay for it to go on in front of a 5 & 3 year old. Come to that OP, I have to say - and I don't mean to upset you any more - I wouldn't stand for my daughters being exposed to it. I'd bloody well chuck those games if DH wouldn't be responsible about it.
Anyway, I think the key here first and foremost before you get on to the practicalities is a united front over house rules - and DH has absolutely got to buy into that - and a firm limit on the Xbox, you set a time and a session length and that's it, providing the game is suitable for both SS and the girls. If you feel confident that SS is less likely to play up and/or damage stuff - because he's had the house rules laid out very clearly to him - then that in turn might make you feel happier about some of the "solutions" re: bed swapping etc that have been suggested. If you can't partially address this by gaining the co-operation of one or both exes then I think the "best" answer probably is that SS sleeps in your room while you invest - if you can - in a decent sofa bed for you and DH whenever SS stays. I know that's far from ideal but you'd at least be able to go to bed when you want and SS would also get some space of "his" own when he comes. Obviously, as I keep saying, you agreeing to try that would only be possible if you felt SS could be trusted to behave and once more, that again comes back to DH stepping up to the mark and doing some actual parenting. In fact, the snidey remarks about a lack of parenting skills apply, IMO, to him and not you.
I really hope you can get through to DH. His agreement and action is key to living through this as best as you can. I do appreciate some of where you're coming from ...... I spent just over a year having to accommodate 4 children in a 2 bed flat and it was hellish. We had the 2 youngest in with us and the other 2 shared but it was like playing a human jigsaw and you couldn't step out of bed without treading on someone. Obviously it also meant privacy was compromised and I found that very hard - thankfully we were able to move somewhere larger and vitally, there was never any question of one child monopolising the living room or getting away with murder. I think it would have finished me off had that been the case.