US citizen Hong Kong to Xian

I am confused on the Country A and B requirements with the 72 hour visa free transit. We currently have booked airfare to Hong Kong in April 2015. We wish to fly to Xian for 72 hours during that visit. Can we use the 72 hour free transit or do we need the L tourist type? I understand we do not need it for Hong Kong as we are visiting for only 9 days.

Is it an option to return from Xian back to Hong Kong immediately before our departure to the United States? If that is acceptable, what is the acceptable layover time?

Thank you in advance!