Sorry it's taken me so long to come back to you on this.
The exclusion does not have to stand. The question is "as a result of the exclusion has the child suffered some form of discrimination". From what you have said here, and I haven't read the papers, I would suggest that there has been. There could well be therefore a claim for discrimination under the Equality Act.
There is no need to go before the governor's disciplinary meeting because to be blunt and without being disrespectful to any governors who may read this, the vast majority of governors do not get the Equality Act and to see their duty as supporting the headteacher and teachers, ignoring their duty to the child.
Although the exclusion may have been withdrawn there is still no reason why you cannot speak the governing body, or more risk realistically two or three members of it or you could ask to speak to the chair of governors and explain your concerns.
If a child has been subject to an exclusion the exclusion remains on the childs record, i.e. cannot be expunged. What should then happen is that the headteacher enters a clear and unambiguous letter, copied to you, on to the file explaining what has happened and that he regards the exclusion as having been rescinded.
The emergency annual review was the correct process to follow. What should of course have happened is that the LEA should have taken responsibility for the child and put in additional support. Perhaps one might also want to ask the local authority why they failed to act more aggressively and supportively of the school.
Hope this is of help