Sorry have not read ALL the posts, only some.
There does seem to be a belief, strongly held among many, that nut allergies are the worst you can get.
My son has milk, egg and nut allergies. I generally only mention the milk allergy - 'tis the worst one because milk is so hard to avoid. The amount of people who have said to me "oh well at least he hasn't got a nut allergy". They are astounded when I say that he hasn't. They have been led by the media to believe that all nut allergies are so bad that even nuts in the atmosphere can cause instant death to anyone with a nut allergy.
This of course is not the case. Some people ARE so allergic to nuts that nuts in the atmosphere, or in an invisible smear of nutella on a desk can cause a life-threatening reaction. But most are not. And some people are so allergic to milk that the smell of a pizza cooking, or an invisibe smear of dairylea on a desk can cause a life-threating reaction. But most are not.
If this child is very seriously allergic then the head is right to enforce a ban.
In my son's school there are children with nut allergies, but he is the only one with an epipen (for milk and eggs). When we were all asked to bring in food for the christmas party, we were requested to make it nut free. DS1 of course had to bring in his own food because no one could ever expect anyone else to go dairy free so that he could be kept safe, that would be totally .
So IMO yes perhaps you are being unreasonable, but I totally feel for you.