Answered by ZUPAN | Nov. 17, 2019 05:16
Actually, your situations is very clear :)
You will print out your cruise and flight tickets and have those printouts with you. Also, you will declare at check-in in Detroit that you will travel to Nagasaki (your first stop on the cruise) via Shanghai using 144 hour visa free transit. After a short inspection of your tickets, you will be allowed to board the plane.
Upon arrival in Shanghai, you will approach 144 hour visa free transit where you will fill out the blue form. You will present your cruise ticket and shortly, you will get a sticker in your passport and be allowed to exit the airport. You will use your time in Shanghai area as you wish until your cruise departure.
When your cruise departs toward Japan, you will not have anything to do with mainland Chinese authorities because Hong Kong has separate immigration and most of other policies and US citizens don't need a visa to travel and stay in Hong Kong at all. After your stay in Hong Kong, you will take your flight toward Detroit, but you cannot take a direct flight because there is no such.
In case if your itinerary from Hong Kong to Detroit has a stop in mainland China, this is again absolutely eligible for visa free transit because Hong Kong is considered as a third country for visa free transit purposes.
As you can see, you don't need a visa and you are totally ready to depart. Also, March is not so distant future :)