The other side of the tunnel actually emerges from Citibank Plaza, and you can either :
(i) drive downwards via Garden Road and route towards Admiralty via the fly-over on the left
or
(ii) drive upwards via Cotton Tree Drive and use Kennedy Road towards WanChai.
Yes, you can. If the security guard stops you, just take out your camera and start filming and then ask him again, that is a public road and he must let you pass!