As a novice, I would like to create a small breadboard with 5 items on it. 3 toggle buttons, and two constant spin (ie. no stop at 360 degrees) knobs.
Those knobs are called "Rotary Encoders".
Is then like the bottoms to send what ever key stroke (single key pee click) to the pc as if it was from a keyboard (maybe even a shift+key?)
Then you ...
There is no "built-in USB" on the Nano. There is only the FT232RL chip which talks USB. That chip is connected directly to the TX/RX pins inside the board.
You are already communicating over the TX/RX pins through the FT232RL to the USB.
Yes, you could use your own FT232RL (or FT232-like) board to communicate between USB and the TX/RX pins, but ...
The twisty click per degree thing is called a rotary encoder.
You can already send serial commands to a computer, so to keep it simple, I'd try to find a way of turning the serial commands into a keyboard click.
If u use the VIN pin of arduino, You shouldn't have a problem , as arduino selects it required voltage automatically, it's voltage regulator senses the voltage from USB and External source, if the power of the external source is higher than usb, the the power is bypassed to the External power. But don't know if you are using the 5 volt pin to turn on the ...
In the arduino ide, go to file > examples > arduino as isp.
Might be a good starting point.
Seems like a lot of work though. If I were in that situation then I'd either use an ethernet cable and SSH, or, use a mobile phone as a WiFi hot-spot.