Assuming your shielding is mostly for EMC, use the simplest option. Everything in one shield, and the shield connected to Earth.
Having multiple zones only creates additional leaks. Any shield not connected to earth is an antenna, i.e. worse than useless.
Your isolated part can share earth with the rest. Just keep power, ground and signals isolated. There should not be any functional current in the earth-connected parts anyhow (i.e.: both parts of the design should have power and ground isolated from earth)
In some situations (i.e. battery operated stuff, car stuff) ground is already connected to the outer metal casing. That can be OK, as long as you don't use the casing as the ground wire (or know what you are doing). Or the thingy uses an isolated supply, with the low voltage ground side connected to Earth (e.g desktop PC's). That's OK too, but don't use the case as ground wire.
As to the single ground connection: Forget it. Just make sure there is a ground wire (plane) next to each power or signal wire (plane/trace), and for any interesting frequency the path through the shield will have too high an impedance (since its loop is bigger) than the path through the wire.
For DC (of significant amperage) you might want to ensure your intended path really has the lowest impedance... But high-amperage DC applications tend to have the + and - isolated from earth, if not for EMC then because a short can be really awful.