USA-Guangzhou-Hong Kong

2 Questions please.

1. Is Hong Kong considered a Third Country when arriving in Guangzhou from the US and producing a flight ticket 2 days later from Guangzhou to Hong Kong?

2. If I change my mind and cross over to Hong Kong by train or car before the 72 hours are over, without leaving the Guangdong region and canceling my air ticket from Guangzhou to Hong Kong, is that OK if I'm in Hong Kong before the 72 hours are over, or do I need to travel by air from Guangzhou to Hong Kong?