Shellfish allergies are varying in sensitivity. Mine, unfortunately, I can’t eat anything cooked in oil that had shrimp. I’ve had 2 reactions from food cooked in that oil - fries and house made tortilla chips. Both told me it wasn’t an issue. One of them admitted their mistake after I had a reaction. Have found only a handful of restaurants that have fryers dedicated for shrimp/fish. Seems the fryers are a “free for all”.
As far as causing flavors to mix? Most of the restaurants that don’t have dedicated fryers tell me not to worry as they change their oil frequently. Doesn’t matter for the allergy, maybe taste contamination it does.