Switchable serial bridge (Bluetooth or USB -> Telemetry)
Look at the Arduino UNO Rev 3 schematic. They have a similar setup with a pair of series resistors on the RX and TX between the USB IC and the main microcontroller, with a direct connection to any shields. This allows the shield to be the primary serial connection. So add a resistor between the Bluetooth IC and the telemetry IC. The USB IC will take precedence. 

As for power, the Uno Rev 3 uses a comparator and a P-channel MOSFET to only allow one power source. Not completely discrete but close enough.