SFO to Guangzhou to Hong Kong

Planning on flying from SFO to Guangzhou on China Southern, stay for 3 days at my grandmother's in Changping City in Dongguan, and then fly from Guangzhou to Hong Kong on Cathay Dragon. Question is, what exactly do I need to show them when I arrive in Guangzhou to prove my onward flight? Just a printed out confirmation email from Cathay Dragon? Also, is it for certain that I can stay at my grandmother's and not have to stay at hotel? China Southern's website states must stay at hotel and show confirmation. Thanks!