The VCC pin represents the StickDuino's IO voltage and is selectable between 3.3 and 5 volts using a small solder jumper immediately beyond the USB connector. By default the jumper is set for a 5V VCC.
The 5.0 pin represents the 5V supply which is either used directly as the VCC voltage or fed into a 3.3 volt regulator (on the FT232RL chip) to create a 3.3V VCC.
Yes!
Connect your external 5 volt supply or regulator to the VCC pin.
Yes. A diode is installed between the USB connector and the board to ensure it only uses USB power if the external VCC voltage is lower than the USB voltage. USB is nominally 5 volts but is often closer to 4.5 volts.
The surface mount ATmega168 parts have more pins and have 8 ADC inputs. The through hole SPDIP parts used in the Diecimilla and NG only have 6 analog inputs.
If you look closely, you'll find that they're VCC, RESET, and GND followed by 13, 12, and 11. Collectively, these 6 signals are necessary to program the ATmega168's bootloader. The single row of pins is much easier to reliably make contact with than the double-row header typical of ATmega ICSP connectors.