H.W.
Hi J.,
Your post was multi-faceted, but a few things popped into mind.
How educated on your daughter's allergy is your family? I ask because sometimes, our friends and family don't always "get it". Something I've noticed myself is that I have to ask/re-ask when one of my son's friends comes over. She's got an allergy to nuts and I can never remember off the top of my head, so I ask her mom every time before she is dropped off just to be safe. ( I also wipe down everything a second time, and make sure my son doesn't eat nuts a couple hours before she's due to arrive, etc.)
You say " I wish we had just not gone now. " Sometimes, when our kids have severe allergies, this is going to have to be an option, whether families understand or not. I know firsthand how hard family events are: I am allergic to lactose and feel like a detective, because I have to ask about everything. I now take my own food.
It may have been more than your daughter could bear, too, to see other people getting to have sweets/cake and not be included. This might have been another reason to gently suggest to your family when they ask why you couldn't come. "It's just too hard for our girl to see everyone eating cake, and not get some." Another idea would have been to leave a bit before the cake, but discreetly.
One thing I'm going to throw out here: I think a support group for parents of children with severe allergies might be a good place for you to seek some insights and advice. I've noticed a few of these sorts of posts recently here on Mamapedia and want to say that while moms would like to be supportive, I've seen some advice that is more about the emotional dynamics between the adults in the situation than it is about the very serious allergies of the child. There is a myriad of issues you've addressed in your post that I think other parents in your situation can relate to, wheras many of us just can't, and might tend to become judgmental. That's why I think another group or forum might be a safer place to take this question.
I hope you find some peace about this.
H.