I am sorry, I have read some chunks of this thread but not all. Some aspects of the OP's situation match mine with DS when he was 6, and I just wanted to post from a 'with hindisght' perspective.
DS, in Year 1 at a small village school, became extremely anxious. The root cause of this was around chaos in his classroom, a teacher not really in control, a lack of peer group and quite a bit of serruptitious bullying (hindsight - not aware of all those things at the time). He developed many traits that are associated with autistic spectrum disorders, to the extent that he was assessed by the Ed Psych - and tbh throughout that period and for years after, I would say that DS had ASD traits or was on the ASD spectrum. He also became a school-induced selective mute, only speaking within the family unit and then with hesitation and a number of peculiarities in the way he spoke.
Like the OP, we reached an 'end of the line' moment, and deregistered him. The headteacher felt that he would never be able to return to mainstream education. I come from a deeply conventional family, many employed in education, and there was quite a lot of indrawn breath.
However, unlike the OP, I did have a clear 'exit strategy', and that is what i think she needs to think about now. DH was moving jobs, and although it wasn't far, we decided to move. My intention was to select an appropriate school for DS, and to 'mend' him in order to re-start school there after the move. Even if that hadn't worked, my long term aim was always for him to be school for secondary (specialist subject teaching is important to me), so that set a) an absolute maximum timeframe and b) an educational approach - we were structured, NC-based HEdders, though this left LOTS of time for other stuff too.
Actually, we loved it. It was a golden period - DD's last year before starting school, DS's mending. It is why I teach today, as a matter of fact.
However, I was always clear with DS that 'school at home' was 'until we move'. He came with me to see all the possible schools (he filled in questionnaires and wrote reviews of them for English tasks). He first spoke outside the family to the head of one of the schools, who was prepared to wait as long as it took (5 minutes +) for the answer, and was capable of making himself so unthreatening for that purpose that DS did manage to speak to him. We selected that school.
DS went for a visit before we moved, and started school again on the day we moved, a few weeks before the end of Y1 so he only had to manage a few weeks. The first day I was a total bag of nerves - had he come out without speech again, i would simply have turned tail and run back to HE. But he not only spoke, he shouted and laughed and thrived.
He's a teenager now, at secondary. No long term scars either of HE or of the horrible period before it, though the traces were there in his speech until at least Y5 so it was a long healing.
I suppose what I'm saying is that this may seem dramatic now, but may not seem so in the longer term. However, the OP does need to have at least a vague exit strategy - long term HE? School for secondary? Return to this school once mended? Return to another school if mended? - before setting out. Oherwise it is just a reaction, not a plan.