Welcome to the board. :)
I'm so sorry to hear about your daughters allergies--but kudos fo you for working so hard to keep her healthy and safe. I myself have a severe food allergy (to, of all things, an element of meat!) which I was officially diagnosed with in Jr. High. It affected me before that though, as the allergy was discovered by cycling through pretty much every sort of restricted diet you can think of starting when I was four! The reason I share, though, is because my mother often expressed the same sort of frusteration when it came to other people understanding my allergy. (It's not every day that you meet a girl allergic to meat! ;0) ). As a matter of fact, the only people who really take my allergy seriously are those who have seen my reaction to these foods. I understsand, as best I can without being you, how frusterating it is. Hang in there and keep putting your foot down! You sound like you're a great mother doing your very best for your daughter. :)
As for those difficult family gatherings, perhaps having a "special" treat for her that she is not allergic to is an option for those times?? She sounds like she is allergic to quite a bit, but perhaps you can find a sweet at a health food store that she can tolerate as a substitute to get her mind off of the cookies and cakes that are so appealing. I know the store I shop at has these flat pieces of pressed fruit (similar to "fruit leather") that are very sweet, made only from fruit, and I don't remember seeing a warning about nut contamination on it. *shrugs* Perhaps cut into small pieces, it could be an option?