The flight attendant should have offered to sit on a jumpseat while you enjoyed the mixed nuts. Her behaviour was outrageous. She could have easily moved back to her seat next to the OP once the OP finished the nuts.

I second Major G's comment about physicians and science; the FA's assertion that the OP's mixed nuts were too dangerous to allow is junk science, especially since it doesn't sound like the FA was concerned about the person across the aisle or the passengers immediately in front of or behind the FA.

Additionally, the seats and carpet are cesspools of mixed nut parts and dust, and the if proximity to nut parts and dust were a serious problem for those with allergies, there would be many more cases of anaphylactic shock on UA and AA flights.

When well-meaning individuals sell their junk science with "do as I say or I may die" (I'm paraphrasing here), logic and science are discarded in favor of illogical bombast.

A brand-new flight attendant (she starts work next week) and already elevating her views (junk science that they are) above the customers. Not a favorable trend for the future.