What exactly is it in shellfish that people are allergic to, and is it possible to extract those chemicals from them?
Allergic reaction to shellfish is typically caused by the protein tropomyosin. Since it is present in basically all of the muscle tissue of shellfish, extraction is not possible outside of a chemistry lab. Said lab extraction would destroy the food and likely render it inedible.
I'm allergic to most shellfish and fish, but there's something that's part of the canning processes that has let me eat canned tuna fish. (I've been allergic since I was about 2 years old and have been eating canned tuna for many years now.) I'm guessing the similar processes in making things like fish sticks or canned crab or other heavily processed food products may let me eat those, too...but I don't like to gamble in this arena.
The process of canning tuna or salmon sometimes changes the fish protein enough that fish-allergic individuals can tolerate these canned products.