بسم الله الرحمن الرحيم
(1) According to Hanafi Fiqh only fish (with its entire species) is halal among all the sea animals and no other animal is halal. However, as per other Imams almost all the animals of sea are halal. Each one has his proof from the holy Quran and Hadith which are written in books in details.
(2) It happens in case there is contradiction in proofs, it is not strange. And in such rulings taqlid is needed as every one cannot reach to the right ruling on his own. In such rulings deep literary discussions take place.
Allah knows Best!
Darul Uloom Deoband