But our lives weren't also in danger.
I still think that the OP, as a competent adult, should have handled this on her own. To need her father to intercede for her sends the message that she cannot handle the job or her own health on her own. It marks her as a problem employee, with an unpredictable and unreasonable family (I am not saying the Dad was unreasonable for being upset, but I do disagree with his discretion in intervening for his adult daughter at work - if it were me, I would be mortified). Since the OP wants to fix the apparently erroneous impression her employer has of her that she is not competent, I think the OP would be better off not having her dad step in for her, which, IMO, really reinforces the negative impression the employer apparently has.
To cross topics briefly - if OP had a potentially fatal dietary allergy, I think most of us would expect her to be able to manage it on her own at 23. To know how to avoid hazards, ask for ingredient lists, manage her expectations when out in public, and stand up to anyone who said "No, really, you *have* to try out this cookie or there will be negative consequences". It is very difficult managing illnesses in workplaces and in public, but still if my ( hypothetically allergic ) 23 year old daughter came to me and said "My boss *made* me eat a peanut butter cookie", I'd be plenty angry at the boss, but I'd mostly feel the have a long talk with my daughter about the "I'm sorry, that won't be possible" line, and when you just need to turn around and leave, whatever the consequences. If your life and health are at risk, you *need* to learn to say no.