Common food allergies re very difficult to get waivers for.
That said, since your daughter has never been formally diagnosed with a food allergy, it would be something that I would want to know the complete truth about.
You should have your daughter write a letter explaining in detail what symptoms she has when she ingests the food, and exactly how she found out that she was allergic to peanut butter. Submit it to DoDMERB. If DoDMERB does not remove the disqualification they will forward it to the waiver authority.
The following suggestion is just that, a suggestion. There is no guarantee that it will work, and you will bear the cost of any evaluations or testing.
You could get your daughter tested by an allergist for common food, to include peanuts and peanut butter. If all testing comes back negative, DoDMERB should remove the disqualification, and if they don't the evaluation(s) will be forwarded to the waiver authority who will most likely waive at that point.
If getting an evaluation is not within your means, you can always wait to see if the waiver authority will request any additional testing, which would be paid for by the government.
If you have any other questions please feel free to let me know.