SkyDyno:
3. I thought the Arduino could run from 12v (supplied to VIN)?
Sort of.
Only if you do not expect to power anything from the 5 V or 3.3 V pins, or more than a couple of LEDs on the data pins.
No relay modules, no display modules, no shields ...
If you anticipate doing any of these things, you need a "buck converter" module to provide 5 V.