VFR flights in HK is actually not so different from VFR flight in the other places. Less controlled and more fun..
1. In the uncontrolled airspace, the ATC would provide Traffic Information service. Separation with traffic and obstacle is ur job as a VFR aircraft.
In the controlled airspace, the ATC would take care of the separation between u and the IFR traffic. U still need to take care of the obstacle separate.
As always, if you can maintain visual separation with the other VFR/IFR traffic, you can make their job a little easier
2. Clearances and Instructions are given in controlled airspace.
Because they are given in form of a clearance to fly to the VFR waypoints and a clearance to join via the VFR routes - all in the HK AIP of course
You take care of the navigation (heading, track etc.) and you are going to have some local knowledge (but i guess it's just like any other places.)
3. No ideal what is that, maybe someone from the ATC can tell more?
Finally
My understanding of the clearance is that they are happy to let you climb to whatever altitude you like (remember less controlled, more fun)
But please do not climb to the airspaces that are used by the big jets.
Long answer short, if you climb above 2000' AMSL, you would most certainly go into the big jet airspace
unless you are in the NE part of HK, where the upper limit of uncontrolled airspace is 3000' AMSL