You will definitely not be able to get to HK by road if you were diverted to Shenzhen. This is due to the authorities being really strict and rigid with the rules on whether the passengers could even deplane or not like their lives depend on it. It's like they will die if it goes against their way. Not too long ago a not-so-lucky flight got diverted there and they ridiculously made everyone onboard to cramp into their seats for 17 freaking hours. Bet these rules of caging passengers onboard jets would be different if someone who couldn't bear with staying In a metal tube for 17 hours straight died on that occasion.