Often when a child starts through the diagnostic process (of any sort) they look to see if over indulgent or neurotic parenting is to blame.
They don't admit that the 'system' doesn't work and not all medical needs are picked upon.
But that shouldn't have mattered they should have taken care of your DS's personal hygiene, i remember the excuses from the original thread, from the school, there was no excuse for not asigning him a TA and making an adequate IEP.
Your son soiled himself that should have been dealt with, the head has experience and knows the guidelines, it is not for a parent to know where to turn to for help.
Have you got your points written down, it helps.
Keep calm, if any professional that you don't know attends, try not to treat them with suspicion.
It is early days but this will be sorted out in your favour eventually. There personal opinion of you doesn't matter, i'm sure you could tell them a few home truths, but let it go and concentrate on getting through this bit by bit, you are moving forward, at last.