The situation is really complex OP.
My ds is older now but around this age he became very addicted to a particular online game. And when I say addicted, I really mean it. It consumed him, changed him and it was the only thing that was of any importance to him. I am a single mum too and so didn't have a lot of male irl support.
I did do the cutting off the internet etc but he got clever and hijacked the neighbours wifi, would claim to be staying over with a friend to use their access etc etc. He started failing at school and just was a completely different kid.
We had to have a drastic change, which involved the whole household. I changed the WiFi package so it wasn't unlimited, so I could use when I needed and changed the password often. As he could hack the password easily.
I enrolled him in a sport which is quite male oriented, he wasn't overly keen at first, but I spoke to the head and explained that I needed help with him (which in itself was a major thing for me to ask for the help), the seniors of the sport were all older. This really was the turnaround for him, as they just took him under their wings, and invited him out to events. I'm a nutshell, he needed a father figure. He opened up to them thank goodness and within 6 months his priorities changed.
You say he enjoys cadets but has no intention of joining the forces. I think this would be ideal for him. Is there anyone at cadets you could speak to and let them know what difficulties you are having with him. Maybe someone there could guide him. They need input from someone external to the situation. You can talk to him until you are blue in the face, it has to come from outside, where that person isn't so easy to manipulate. He knows what buttons to push to get what he wants from you and as hard as it is to hear, you cannot do this alone.
What about some type of gap year after he's finished school? Something to aim for. Maybe Camp America or even getting all his qualifications in place to go work abroad, maybe on one of the super yachts. You say he's been to Africa, there are a lot of programs for school leavers to look at, which I really think will change his outlook. And it gives him something to look forward to. Right now, what does he have to work hard for? He knows he's going to get chucked out his home in a few months, knows that he has a deadbeat dad that he may or may not think he takes after. His life is that Xbox and the friends on there he doesn't even know irl most likely. It is a short term gratification that he is getting from it. He needs help for something more long term, something to look forward to.
As for the money for the chores. I disagree with him having to be paid to do these things. He should just be doing them. He needs to learn that someone isn't going to clean up after him forever. Set the chores, but don't pay him to do them. If there are no plates left as they are all in his room, then he doesn't get a meal on a plate. If he doesn't wash his clothes or put them in the basket (whatever your arrangement is) then he doesn't have clean clothes. Stop enabling this. Yes you may been out of your norm, and it is a bloody inconvenience yes, but you have to get out of your routine too, and make the changes necessary for him to see this.
It's easier to shove things under the carpet to keep the peace, but long term it's not helpful.
I wouldn't sell the Xbox, but I also would not be getting him a high value gift for his birthday.
Can I just say it took a while, but my ds is through this know and is actually a nice human again. He wasn't nice for a while and it's hard for me to even admit that. But the issues were much deeper that what showed on the surface.