This is really hard. I'm so sorry you are where you are, and it's normal to struggle when a child displays behaviour that reminds you of an abuser, especially when there is a blood link to your abuser.
But. But. But.
It's totally normal for children to try to manipulate you. It's part of growing up for kids to do that to their parents, really. Mine does sometimes, and he's autistic so really he does it massively less than neurotypical children do. It will trigger your anxieties and rightly so - but it's not because he is abusive. It's because you have been abused in the past and so he is treading on a nerve. Learning how to interact with other people and to balance their needs and wants with your own, and what is healthy and what is brattish, takes years, and it's normally a work in progress for all of us. An 11 year old is going to be a petulant, angry, manipulative brat sometimes, and that's age appropriate. There's a reason they won't diagnose a personality disorder until someone is in their 20s, and it's because to a large extent that sort of extreme behaviour is not uncommon in children and adolescents. The problem is when adults still do it, but he has a good decade to go yet, and there's no reason to think he will, with love from you and proper support and help from outside for you both.
He sounds like he has massive, massive separation anxiety. I can't put my two to bed alone either because DS can't be alone after supper - he is too anxious. I recognise what you describe and it's really hard on everyone. The night I had no help from anyone and DH was away with work, I ended up with both kids watching TV until they fell asleep, as DS can't be alone and he can't let DD settle. He is the sweetest kid, he isn't trying it on, but he can't do it.
I think one thing to do is to establish what might work, even if it's less than optimal. Is there a tv programme, or film, or computer game your eldest loves? Bonus if you ration it (we have to ration screen time). Might it be doable to let the eldest have that time while you put the younger to bed - I know screens before bed are bad, and in my son's case it wouldn't work as it would throw his bedtime routine and then he wouldn't sleep at all, but as a parent who sometimes compromises just to make things function, I think there are times when you have to go with the survivable. If there is a workaround that won't keep your eldest awake until midnight but will allow you to settle the younger, then go with that.
The other thing that has really helped DS has been put in place by his school: a mix of CBT type work from a family support worker, and mindfulness put in place by the SENDCO. If you are working with SS, what sort of emotional and social support is he getting? And in terms of exercise - DS finds trampolining a big help as it increases endorphins and expends energy, plus is a sensory release. I think exercise can be really important for an angry, agitated kid, or adult for that matter. What is he getting - again, are there agencies who might help with provision, if it's unaffordable for you?
The Explosive Child book is excellent, agreed. But I do think it's essential that, as others have said, you look at him as a child. He is going to behave in extreme, and at times obnoxious ways, because he's a kid growing up and you are his mum. That doesn't mean for one moment that you should accept it, and you are doing all the right things in setting clear boundaries and refusing to allow him to get away with it - he won't want to get away with it; he needs to know he is safe with his mum in the driving seat. But please, please, don't think he is abusive because he is troubled. What you describe is a child who has already had to cope with more than anyone ever should. Think what harm and pain your ex did you - and you were an adult. You could, and did, leave. Your son has to cope with his whole life being not just intertwined with your ex, but being the result of your ex - he's his dad, not a former partner, and he's an arsehole. That is a huge thing for a small person to live with. I think it's a very good thing that his father is out of his life. He sounds like he has done quite enough harm already. I don't buy the idea that a man who abuses his partner is somehow going to be any different with his kids. That's the kind of thinking that led a judge to return Ellie Butler to her sperm donor. It's a good thing, I think, that your ex is now out of the picture... but it is probably going to make your son angry, and as the parent who loves him, and he knows loves him, you're the one he will punish. Horrible, but true. I'm the one DS beats up when he has a sensory-triggered meltdown and it isn't because I create the neural overload, any more than you have your son's sense of abandonment. We're just the ones they feel safe offloading on.
Have you had family therapy - have you asked for some? It does sound like you and your boy could do with some more support.
You're very brave. I just wanted to say that. You're having to parent a very challenging child when that presses triggers from past abuse, as a single parent. It's got to be very, very tough to do that. Please get the help you both deserve and need. 