My kids have gone to both peanut banned and peanut allowed preschools/schools over the years. (I am severely allergic to crab - but no other seafood thankfully! same as my dad, and assume my kids are until we either get them tested or they try it when they are much older and near a hospital... Luckily, crab is so uncommon that it is not hard to avoid so poses almost no risk unless we are a hoighty toighty wedding reception or something like that.)
At the schools where it was banned, you could never assume that all would comply! There is really no security in a peanut ban IMO. What about the child who had pb toast for breakfast and washed his hands even, but not before wiping them on his shirt? We are talking about kids. I once brought orzo salad to a toddler group potluck lunch at school (with kids) - full of pine nuts. I knew about the peanut ban but didn't think about pine nuts being a problem - and am not sure if they are or not, but by the time someone pointed it out, it was on a dozen plates all through the room. Our class didn't have an allergic child; it was just the policy of the school as there was a child in a different day's class that had an allergy. But my point is that even those who are trying to comply can make mistakes/forget/not fully understand.
The way the non-ban schools did it was to be sure all the parents and kids understood who had allergies, what to avoid, and what to do if there was a problem. One preschool mom of allergic to everything under the sun child (poor sweet boy - he honestly was allergic to almost everything) was incredibly good about explaining it all to us, sending him with his own snack and plate to eat it on, and teaching her child not to eat anything that she didn't give him. He was three at the time - and amazingly good about it. She never once asked us to change what we fed our kids at school or at home (I CANNOT imagine schools trying to ban what we feed our kids OUTSIDE of school), but just educated us and her child. She had such a wonderful attitude about it that we all did try our very best to watch like hawks. Nine months of preschool and never had to use the epipen or call 911. No one in the class had any issues with his allergy as it was handled so well. We all were great friends - moms and kids - and continue to be in contact to this day, nine years later.
Another preschool (dd2) had a poster on the wall with pix of the allergic kids, their names, which class they were in, and what they were allergic to. Teachers, parent helpers, etc. could just glance up and see. No issues all year there either.
I do think that although it would be ideal for everyone to "go the extra mile", it is not realistic to expect it. When my dd1 was five, I didn't think it was safe for her to stand on the corner of a busy street, alone, waiting for the bus. I didn't trust that she could keep herself safe, stay off the road, fight off someone who might try to take her, etc. I had the choice to either stand there with her (my responbility as her parent) or just drive her myself. It was inconvenient but I drove her. I don't think I could ask people not to drive on that road during the times she'd be there, or for others to take turns waiting with my child - she is my child; it is my issue to deal with. Others being helpful is wonderful, but it falls on me to go the extra mile for my child.
I think coming at it from an education standpoint instead of a ban standpoint would end with more compliance and a safer environment for those with allergies. I think what happened with the PB on the swing set in your yard is AWFUL; I am so sorry!
I also am curious as to why so many kids have life-threatening allergies these days. I don't remember ANYONE with these severe, breathe it and die type issues all through elementary, jr high, high school, college. I don't remember ever reading about anything like it either, or on the news. Now it seems so common - why? When it is just a reaction to INGESTING dairy, wheat, PB, etc. it seems easy enough to teach the child not to eat what has not been previously approved by his/her parent. It seems the touch/breathe component is what is causing all the issues. What has happened over the last 20 or 30 years to cause all this?