transit visa 72 hours, 2 questions

1) I land in Beijing on 9 oct, 11:20am, coming from Tokyo. I would like to apply for a 72 hours free transit for visiting the city and see somme friends. Is it possible to take off to Cambodia on Sunday Oct. 12 at 4:30pm if I pass through the border before 11:00 am and then stay in the transit area of the airport? I already booked my flight to Beijing but not those to Phnom Phen, waiting for this important information.
2) I transit again in Beijing for returning to Europe. May I get a second 72 hours free transit on oct. 20 before taking off on 21 at 1:30 pm?
Thanks for your help!