technical question about a Chinese visa

Hello,
I am currently in Europe and have not gotten through to a representative to answer the following time sensitive question, so I am writing to you for resolution. Thank you
I am a USA citizen with a valid passport.

I am traveling for pleasure from America to Shanghai
on one ticket will be in Shanghai for 70 hours
I leave shanghi to India and connect in hong kong for 5 hours on the way to india.
I am travelling through shangai as the tickets were the best price.

I was told that I do not need a visa as I can stay in Shanghai for 72 hours.
Is this correct? Do I need to get a 72 hour one at the airport in Shanghai? or what do I do?
If I stayed over night in hong kong for any reason, can I do this?
Will I need one or special document to stay in hong kong if for any reason I had to stay overnight, such as a missed flight connection or some other reason.
Thank you.
Sincerely,

Donald Epstein
donnyspersonalmailgmail.com