Restaurants in Texas, at least, require that all sushi fish be flash frozen, and kept below zero for 3 days. If it is salmon, the require 6 full days. If you do not do this, you simply put a warning on the bottom of your menu with some severe disclaimer. I would go with something like "eating raw fish is poisonous and will straight away kill you. Eat at your own risk."
If you "sear" the tuna, and the internal temp does not hit temp required to kill bacteria, you are still exposing yourself to the dangers. Traditionally, sushi is consumed with things that are basically digestive aids--perilla leaf, ground wasabi, pickled ginger, green tea, sake, vinegared rice, etc. Salmonella is not something you will catch from inside the fish, it is caused by poor hygiene of the people handling the fish meat.
When I am in a good grocery store, I look for various things in raw fish--one is smell(or lack thereof). It should smell like nothing--if there is a LOT of meat, the air will smell slightly crisp, but never fishy. Whole fish should look attractive, good colors, clear eyes, and a healthy build. The meat should look bright, sturdy, and dry, with a consistent quality. If it has been trimmed unusually, either is had bad spots, or someone is cutting it that doesn't know what they are doing(and therefore I do not trust them). The fat should have good color as well. Then I pick based on taste(fatty/less fatty etc). Basically, between keeping fish as a hobby and cutting sushi fish all day, I just go into a store and see beautiful fish, recently living, and meat that looks like I'd be proud to serve it, and I eat it.
But I still advise you eat all your uncooked sushi and sashimi at a trustworthy sushi bar(preferably a busy one). Sushi at home is more a novelty. As far as not being on the coast, it doesn't matter. Sushi Bars in LA can get fish faster from Tsukiji in Japan than they can from a dock 20 miles away. I worked at a bar in North Texas, and our Tuna got to us on Saturday, and it was swimming off the coast of Argentina on Monday. That's why it costs so much!
The "best possible scenario" you described is very common. In fact, the newer fad of serving raw fish in italian places as "crudo" was popularized by Dave Pasternack in New York, who is a very avid fisherman, and always carries some olive oil and salt on his boat to eat fish raw right after a catch. They just brought the concept into his restaurant. But while the texture might be best, the flavor, as all meats, benefits from a time of resting, which is really just a mild level of decomposition, breaking up the stuff we taste into nutritionally available forms. The Japanese tradition was to catch a tuna, and bury it in the sand for a day, dig it up, and eat it. It's like aging a steak. If your food is kept in the right temperature conditions, and kept clean, it will dry out and/or rot faster than it will grow any deadly virus or parasite.