using galaxy tab as a bluetooth device for mobile
You'll have to understand, that Bluetooth is based on a master-slave structure, and so are it's drivers and protocol stack.

Most of operations you have specified use existing Bluetooth protocols and profiles:


hands free => HFP
music streaming => A2DP
music control => AVRCP
tethering => BNEP
making calls = TCP
contact viewing => PBA


The problem is, the tablet can act as a master for these protocols, but is missing the stack and drivers for slave functionality. You can, for example, connect the tablet (master) to a headset (slave) over HFP, but you can't use the tablet as a headset.

Until someone develops a more complete Bluetooth stack for Android, which includes both master and slave profiles, you can't do this over Bluetooth.