FWIW, I am very allergic to prawns/shrimp, crayfish and langoustines -- but can eat scallops, mussels and oysters with no problem whatsoever. And scampi isn't something I eat much, but I know I ate it as a child with no probs.
Lobster used to be fine, I had it loads when we used to visit family in Maine when I was a kid. But when I had some here about ten years ago I felt a bit of a reaction, so I've stopped.
I reckon an epi-pen is worth looking into. I have one. My reactions got worse as I got older -- at age 9 I just had a horrible itchy mouth and was sick after eating shrimp, whereas at age 23 I ate prawns by mistake and ended up vomiting and then in A&E with a hugely swollen face and difficulty breathing, and had to have double the usual amount of cortisone (or whatever, I think it was that) injections before I was OK to go home. It really gave me a fright.
I have found that Chinese/Thai takeaway places are a real nightmare for this allergy, so it's worth being extra careful with those. In the past year alone I've had three incidents where veggie and/or pork-based starters turned out to contain very obvious prawns, despite clear assurances from the person taking orders on the phone that they were shellfish-free. It's getting to the point where it doesn't feel worth the risk. I think it's because often, the takeaway restaurants buy things like spring rolls or dumplings frozen and ready-made, and in many cases they are not really hugely familiar with what's in them; or there are mis-labelling issues, or simple confusion. So please watch out for this.
Anyway -- I'm sorry to post at such length, good luck