That fake crab has so many other bad things in it if you're allergic to crustaceans. If all seafood is out, then you can't eat the fake crab either, because they use fish like polluck to flavor to taste sort of like crab. That imitation stuff also has artificial colors and flavoring and preservatives, which people like me have to avoid. They also use eggs and modified food starches with that stuff. There are just so many variables in that fake crab stuff that you should just avoid the stuff when you know you have allergies or sensitivities. It's just too risky, because allergies and sensitivities manage to only get worse, so it isn't safe to risk your health and life with this kind of allergy. Allergy medications won't prevent the allergy from getting worse.
Following are some treatment for your reference:
Dietary avoidance: Avoiding the offending allergen in the diet is the primary treatment of food allergy,you said you seafood allergies, you stop to eat seafood.
Unfortunately, there are no drugs to help with food allergies. The only option, and this is not always 100% helpful, is to have a food allergy panel run, then, when the doctor finds out what you are allergic too, you can have his nurse or technician make up drops that counter-react the allergen. However, this procedure is mainly done through very experienced Naturopaths or natural medical doctors. The only clinic that I know of is the "Tahoma Clinic" outside of Seattle, that is where I had mine done.
This being said, I think the safest route is just plain avoidance of the offending food, that way, you are sure you will not have any serious reactions.