Since MV Diamond Princess is under construction in Japan rather than in Italy, she won't have so far to go to reach the left coast. IIRC, the first voyage of MV Diamond Princess with passengers was from either Bangcock or Singapore to Los Angeles -- which means that she went from Italy to either Bangcock or Singapore without passengers as a "shakedown" for the crew. The cruise from Japan to Seattle would provide a similar "shakedown" for the crew to work out the kinks.
In any case, Princess has not yet released an official announcement of a cruise from Japan to Seattle. We won't know for sure until either (1) there's an official announcement of such a cruise or (2) the ship arrives in Seattle without passengers.