Before I kill myself troubleshooting I'd like to ask if it's possible to power the Arduino via the "Vin"-pin with 12VDC?
As far as I can understand this should be quite doable, but I'm quite the newbie and might have gotten it wrong.
Yes the Vin pin can take 12V. Depending on how much extra current you need for things like LEDs that are also driven off the 5V line the regulator can get hot so while 12V is OK 9V is better.