It doesn't go through the voltage regulator. We assume that the 5V from USB is in range.
In this case, will 5V not be enough to handle my two 3.3V LEDs in series pairs?
I guess you'll see a voltage drop of 2.5 V at each led and a current of less than 20 mA. No resistor required, probably.
There are several web sites and schematics on the web that suggest you can attach an LED directly to an Arduino output pin with no current limiting resistor. They are wrong, and following them will damage your Arduino.
Is this true? In this case, will 5V not be enough to handle my two 3.3V LEDs in series pairs?
Bonus question: As long as only one LED is on at once, I could handle driving all 14 seperately, right?
Do the LEDs really have a 3.3V forward voltage? In that case you are limited to one per pin. I think some external circuitry is starting to look good here.