In sea animals, only fish is lawful to eat according to the Hanafi Fiqh. Therefore, eating crab is unlawful. There is dispute between scholars about the permissibility of prawn; some regard it halal considering it fish while some regard it haram and do not consider it a fish. In view of the difference, it is allowable to eat prawn, but avoiding is better. We do not know what is mariana mix etc, so we can have no say in the matter.
(2) Such thoughts are natural, there is no matter to worry about, you should only strive hard in good action. Actually, it is impossible to know the facts of the other world sitting in this world.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband