Both my sons are anaphylactic to peanuts, tree nuts, and more, and have flown by themselves (they're 20 and 17). They take Benadryl (their allergist has small 4t labeled containers she gave them) and two epipens with them on the flight. DS1 hasn't felt any discomfort to date and has never requested accommodation. DS2 didn't have problems until a couple of years ago, and then just a general bleh, so we asked Southwest for peanut-free flights over this past summer when he flew alone (New England to the Southwest) and that worked out fine for him.
One thing with Southwest did stress was that they were unable to control what other passengers brought on board, which DS could work with. He says the lady next to him on the last leg of his flight home had almonds, but he had no symptoms from being around while she ate them. We didn't need further accommodation, in other words, so I don't have experience with either getting it or having difficulty getting it.
They're both really vigilant about cross-contact and tend to hold off eating until they're in surroundings over which they have more control, or just stick with what they've brought along for themselves. (DS1 likes watching the Fritos bags inflate as they gain altitude

)