Any ocean seafood you get in Minnesota is going to be frozen at some point. A grocery store may display what looks like fresh seafood in the fish counter, but it came to the store frozen, and they thawed it. I have the same situation in Dallas, for the most part.
I don't like sushi, so I don't know how freezing effects "sushi/sashimi grade" tuna, or other sushi seafood.
As for farm raised, that can be good or bad. It depends on where the farms are. Fish farms in the USA are covered by strict regulations in the name of food safety. In other countries, namely China, fish may be farmed in polluted water, or fed unsafe foods.
Lucky for us, The USDA requires all fish sold in the US to be labeled with where it came from, and whether it was wild caught or farmed. I love shrimp, and will only buy shrimp that was wild caught in the USA.
Also, to answer your last question, seafood should only be frozen one time, as soon as possible after being caught/harvested. It should stay frozen until you are ready to use it. If you buy "fresh" fish from the supermarket fish counter, and it has already been frozen and thawed, you really don't want to freeze it again.
CD