I feel for you @drspouse , it's a very difficult situation.
Going by your previous responses, I think you might not like what I'm going to like what I'm going to say, but I say it with the intention of trying to help, and because I think you need some straight-talking.
I have ADHD and so do both my children. One of them has ADHD, autism & GAD (generalised attention disorder), which were all diagnosed at age 12 and up, and the other 'just' ADHD. I've unfortunately been in the same situation has you of having to call the police to come help re my SEN child - my DC1 with autism & ADHD had a mental health breakdown and was violent. We've struggled to find DC1 the right school and they've moved several times, but we got there and they did very well academically.
Professionally, and without going into specific details, I advise schools on their SEN responsibilities, look for and address potential disability discrimination, and effectively at times mediate between parents and the school on how to resolve the situation.
So personally and professionally I've known of a lot of 12 year olds with ADHD and/or ASD, and I have never seen a situation like your son's where the child only had ADHD. Or even only ADHD and anxiety. His whole profile screams autism & ADHD to me, along with trauma in the mix (Or even no autism but trauma, ADHD and FASD, yet you don't think his adoption is particularly relevant..) I appreciate several screenings for ASD have told you he doesn't have it, but we had to go through several assessments to get our DC1 diagnosed with autism. Often the signs don't really click in until puberty.
I find some comments you made about ADHD very absolute - rather insultingly simplistic - or just wrong. I really think that what you're attributing to purely ADHD just, well, isn't. wish to correct these comments, as much for anyone else reading's benefit as your own:
Talking about feelings with a neurodiverse child is also pretty useless. For a child with ADHD of his age, talking therapy doesn't work.
This isn't true. Talking therapy does work with children with ADHD. Children with ADHD as a generalisation love to talk, including/especially about their feelings as they have so many and are often quite emotionally literate about them. Where talking therapy might not work with however is with autistic/AuDHD children, who typically do not like talking about their feelings.
He's 12 and has little insight into his own thoughts. Given many adults only click they are neurodiverse in their 40s, I'm not sure he's likely to know!
Surely you realise that the reason many adults in their 40s only click they are neurodiverse in their 40s is because is their diagnosis was missed when they were younger and ADHD in particular wasn't a thing?
NICE only recognising ADHD in children in 2000, and in adults in 2008? And to be really blunt, the reason why the diagnosis of ADHD in particular continued to be missed in so many people who then realised they had it in adulthood is because many of those people were fairly academic and although their schooling wasn't smooth, it wasn't particularly disruptive. This is not the case for your son. You can't equate his situation with that of adults who clearly masked well and were able to get through school and into work, even if it came at an emotional cost to them.
I don't really know what you mean by "scenarios on the spectrum", do you mean a doctor would assess for ADHD, ASD etc all at once? If you got past the screening for more than one condition, sure, that could happen, but otherwise it would be like doing a full body x-ray for a sore leg.
It's not uncommon these days to pre-screen for ASD when assessing someone for ADHD. It's definitely not like doing a full body x-ray for a sore leg; both ASD and ADHD come under neurodevelopmental disorders.
Any solution that involves us sending him away till he's 18 is just going to have him back here at 18, bigger, more aggressive, and with no qualifications. He MUST get an education even though he doesn't want to.
I don't agree that any solution that involves you sending him away til he's 18 will result in him having no qualifications. I know of several places that provide qualifications, and young people who have done them. They may just not be the qualifications you want him to have at the time in his life you want him to get them.
To get your son the help he needs, as hard as it is IMO you're going to have to loosen your black & white thinking and compromise. At the moment, he doesn't want to learn in school. So I think your focus has to first be on him being happy most of the time in school and at home, with him not getting dis-regulated very often at either. Him learning will then follow, or at least can be addressed after.