Visiting Shanghai for 2 night after disembarking a cruise ship - is this okay for transit free visa?

its OK as long as the last port before Shanghai is not also in China + the next landing by plane is not in the same country as the last port visited

prior port is Hong Kong, so a visa would be required then, correct?

Of course not because Hong Kong is considered as third country in 144 hours visa free scheme. In other words, your itinerary would be, as far as the Chinese authorities are concerned: Hong Kong - Shanghai - US or some other country. Definitely eligible for visa free transit (travel).