I haven’t read the whole thread just your posts OP. Just wanted to say I’ve been through the same thing. My child didn’t settle as a toddler and has had phases on and off since then. Now 17 and still slept in our room once this year when he had an exam the next day and couldn’t sleep. He has ADHD and dyslexia so doesn’t cope with pressure. It’s not typical but it’s not abnormal either.
We initially tried everything and when he was young we were very firm as he has a younger sister and when she was a baby he couldn’t sleep in our room. It made no difference - if we took him back as he got older he’d stay awake until we were asleep and then sneak in!
They don’t want to do this so making them feel ashamed makes it worse. Our attitude developed into if he wanted us to sit on his bed while he relaxed to sleep then we would, if he came into our room to sleep as long as he didn’t disturb then ok, it was better for him to get sleep than not. I wouldn’t sit in his room in the middle of the night as you also need sleep.
its not necessary to get a diagnosis for him unless you think it will help him but you can read up on autism as it helps learn how to help them and also not to feel at fault for raising an unusual child. You can then treat without the baggage of the label.
My child is close to us, has friends but finds it difficult to make new ones quickly, maybe autistic but is functioning. He will just take longer to mature than others and may never like being alone. Hopefully he will fall in love with someone who appreciates his loving nature at some point.
Fighting it, making it out to be more than it is like some have suggested doesn’t help you or him. it’s not the worst stress coping mechanism, lots of 12 year olds are vaping or even drinking alcohol to cope. Just keep talking about it and explaining your viewpoint, if he understands why he should sleep in his room but doesn’t then he needs the reassurance.
I am sure you’ve tried all the sleep hygiene - my son really liked my floating bed or other mindfulness stories but if they are feeling anxious then nothing helps and they get fixated on coming into your room as that’s the solution in their mind. I can’t sleep when I am anxious and either cuddle the cat or husband if he is awake. I’m pretty sure the judges wouldn’t say that’s weird yet for some reason when it is a child they jump to the conclusion that there’s something wrong that should be disciplined or therapised out of them.
I’d go with gut feel rather than what you think you should do. Ignore it if it isn’t harming anyone and continue to be positive about his bedroom and the advantages of sleeping there as hopefully it will decrease naturally without it becoming a big deal.