Is this covered by a 72 hour transit visa?

Hi! We would like to fly directly from Seoul, South Korea to Beijing arriving at 9:30 a.m. on Saturday. Then we'd like to leave on Tuesday afternoon flying from Beijing to Phnom Penh, Cambodia. I'm worried about two issues and hope you can help me understand if these are covered under the 72 hour free transit. First, the flight leaves Beijing at 4:30 p.m., which is technically over 72 hours, but within a 3 day period. From what I've read, this seems to be ok but I want to make sure. Second, the flight to Phnom Penh has a 2 hour layover in Gwangzhou before continuing on to Cambodia. Will this be a problem? Is this considered landing in another city even if we don't leave the airport? Please let me know. Thanks so much for your help!