Not necessary, because the author doesn’t make any predictions about future efficiency and reliability of North American cars. The author’s argument is based only on the fact Japanese drive on the left side of the road. The author believes making cars with right-side steering wheels will appeal to the Japanese more than making cars with left-side steering wheels, regardless of whether American cars continue to improve in other ways.

Necessary, because if it were not true — if Japanese ARE inclined to buy cars with left-side steering wheels even if they drive on the left side of the road — then we have no reason to think that making cars with right-side steering wheels could influence Japanese demand for American cars. The author’s argument requires the belief that driving on the left side of the road implies a preference for right-side steering wheels over left-side steering wheels. This is why the author thinks Japanese are more likely to demand American cars if we make cars with right-side steering wheels.

Not necessary, because the author’s argument doesn’t depend on a belief about Japanese car safety standards. The argument can instead be based on the Japanese having a general preference for buying cars with right-side steering wheels because they drive on the left side of the road.

Not necessary, because the author never argues that making cars with right-side steering wheels will have have a significant impact on the disparity in demand. Rather, all the author concludes is that making cars with right-side steering wheels will “remove one obstacle” to reducing the disparity. In other words, it will remove an obstacle to increasing Japanese demand for American cars. This doesn’t imply that over half of Japanese would, all else equal, prefer the American car.

Although the author does believe that making cars that better meet the needs of Japanese people would be something that can help reduce the automative trade imbalance, the author doesn’t have to think that it’s necessary to reduce the imbalance. Notice that the conclusion asserts that “IF” we do this, one obstacle to reducing the imbalance will be removed. The author doesn’t say “ONLY IF” we do this can the obstacle to reducing imbalance be removed.