I find it very odd that my partner who has a shellfish allergy can't specify that on an airline meal request, yet there are no end of meal options for various religious or lifestyle choices. Boggles the mind. I'm sure when someone gets killed they might change. Until then, enjoy your kosher low sodium bland vegetarian option...
There are several airlines (in fact, I can't think of one that does otherwise) where you can't specify a meal option that accounts for allergies, with the exception of gluten and (somewhat) lactose. There is no way to specify an allergy to shellfish, eggs, nuts of any kind, soy, MSG, synthetic compounds, etc. etc.
When someone is killed I'm sure everyone will wake up, but until then. I can't remember the exact first aid treatment for someone suffering an allergic reaction - I think it is epipen, but I'm not sure how long that can stabilise someone for (i.e. long enough for them to divert to ground and get to medical treatment), let alone if there would be additional adrenaline on board in the first aid kit.
There was one former mainstay member here who had a nut allergy who experienced a reaction on a Trans-Tasman flight to Australia. I can't remember if the flight was a diversion or it actually just hurried up to get to the destination, but the flight landed at Sydney and he was taken to hospital straight after, making a full recovery. The main barrier to his survival was not the airline, but actually the customs officers who were blocking the paramedic staff from offloading him due to border protection protocols.
Back on topic, the main problem I find with serving cold prawns is that they may not taste the best when served on board. This is not talking about allergies. First, prawns go off very easily, especially when subjected to temperature changes. Masking that kind of off taste with a sauce is ruinous and irresponsible. This is why I was turned off BA's shorthaul CE salads with prawns (or shrimp, if you will) - I had one once and I knew that those prawns were off. The crew didn't proffer so much as a quick query as to why I hadn't touched the prawns or the salad, and I have no idea how the other pax in the cabin on my flight ate that salad and didn't get sick the next day.
Next, the whole freezing/chilling and returning to room temperature (not heating) of the prawns pretty much kills the flavour. It'd be basically like eating fishy/shellfishy tasting chew-plastic. The misnomer seafood stick would taste better than that. Again, you could match it with a sharp sauce (e.g. acid based dressings or creamy ones), but then one might have been best thinking up of a better dish in the first place.