LAX>>HKG 3 nights>>PVG 3 nights >>NRT 7nights

Hello, I have two questions really and thank you in advance for your help.

We are US citizens flying from LAX to Hong Kong for 3 nights, then HKG to Shanghai for 3 nights then Tokyo for 7 nights. It is my understanding that we can use the 72 hour free transit for Shanghai right?

On our return flight from Tokyo to HKG, the flight has a stop in Shanghai for an hour. My question is do we need a visa to stop in Shanghai while waiting for our flight? We will not be leaving the airport and are only stopping due to flight schedule before spending one night in HKG before flying back to LAX.
Dee