Oh Claw!
I think you are right, deep breath and take some time to take stock. Keep your energies on what matters.
From memory your GP signed your DS off, at least in part at your suggestion and just before the school holidays - when frankly missing school was not a big deal. So from the GP perspective I can see why the GP is not sticking his neck out on signing him off longer term as the LA will be asking for much clearer evidence as to why and your GP has not got evidence to back up his view. Unless the GP has seen your DS express anxiety about school or seen a pattern of self harm over a period of time then he can't say much more. He has referred to CAMHS. I suppose the GP could say that your DS should be assessed by CAMHS and CAMHS should decide if your DS is fit to attend or not.
Ignore the SW she keeps saying she will do x, y and z and then never actually does anything. It will be obvious from a chronology she only jumped when the SEN team or school rattled her cage. If the LA try and make out they have concerns about you as a Mum then you can point out how their SW did nothing so they cannot have been that concerned.
What you can do is keep asking your GP to record the facts about the sores in the GP notes. Presumably the GP can say when he examined DS on x date he had many sores, and when examined on y date they had mostly healed / disappeared. There is no evidence other than from your DS the sores are due to gnat bites so I would just ignore that - if it becomes an issue you can ask the LA what evidence it / school has that it is gnat bites. A Tribunal will see that DS version of events may not be reliable.
I would not ask your GP to say anything that goes beyond his remit and expertise as that will allow LA to undermine his opinion. Just ask him to continue to record the facts. e.g. it would be worth taking DS back at end of summer if further improvement in sores. Try and keep the GP on side.
Do you have photos from the last tribunal with similar sores? If you have photos of sores going back a long time then that is good evidence it is not gnat bites! You could show these to the GP as evidence that you are not inventing the self harm.
I am assuming you are not going to send your DS back to school? If you did then you would quickly get evidence self harm is linked to school, but understandable if not want to go there.
Is there anyone who can back up the pattern of sores being linked to school? A swimming teacher or scout leader or friend? Any evidence you can collect just in case you need some back up
It is a different ballgame for the GP to sign your DS off school long term than it was before the summer and really the GP or CAMHS would need to dx some kind of anxiety etc. LA are not going to want to pay for home tuition without evidence and you will have to consent to some sort of medical assessment of anxiety. Otherwise they will treat you as HE.
It is not out of order for LA to ask for GP opinion as to what level of education your DS can manage - especially as you brought the GP into it and he was withdrawn for medical reasons - although the LA and GP should get your consent to this. After all you have asked for home tuition and in assessing how many hours is reasonable for your DS to participate in then a medical view as to what your DS can cope with is needed.
Obviously the LA is not asking for advice to work out how much home tuition to give you but to undermine your case! But that does not mean their request for medical evidence is on the face of it unreasonable. Although doing it behind your back is.
I personally would not kick up a fuss now. I would write a very brief FOI request for your DS's GP notes and LA file - you don't need to give a reason but if asked just say you need info so you can prepare for Tribunal. You will then get copies of all the letters between LA and GP, and with anyone else e.g. SS. I would write this as a separate one line letter to the GP and Data Access Officer at Council
School and LA are clearly going to paint you as someone who is subverting the process e.g. making GP sign your DS off school, inventing self harm etc to get your own way over the statement. Thats why I think you have to be careful about being seen as really reasonable. They will want to paint you as someone who is difficult and obstructive and unreasonable so you have to be the opposite however much that sticks in your throat and means letting other stuff go.
When it comes to the appeal you can do a chronology which shows that LA lied about GP (if that proves to be the case).
Do get legal advice if you can and check with them what to do. In terms of replying to the LA I personally would keep it polite and brief. Ask the LA for a copy of the GP evidence they are referring to. Then say that you remain of the view that your DS is not fit to attend school due to anxiety. The sores were caused by self harm not gnat bites and that the self harm stopped immediately when you withdrew him from school and his anxiety reduced. If he returns to school in Sept you expect the anxiety and self harm to start again. You could say that you consider that your DS should be properly assessed by CAMHS, they are the experts in anxiety, and until that assessment is completed you will be keeping your DS at home and expect the LA to make the necessary home tuition arrangements. (obviously thats assuming you are not sending your DS back in Sept)
In a way the LA are stuck as if your DS goes back to school and does self harm then they will quickly be proved wrong and will they actually want this evidence against them at tribunal?
But you have to be super super careful to look absolutely squeaky clean to a Tribunal or SS investigation. Making it clear you are happy for your DS to be properly medically assessed shows you have nothing to hide.
Hope that helps. PM me if you ever want to discuss away from here.