12V supply to Vin Pin on Arduino
The very same schematic was used for the original Uno32 from chipKIT.  As it happened, someone managed to blow it up by applying 15V to the barrel jack.  That was perfectly within the specs of the regulator, but the comparator ejected all the Magic Smoke™ and ceased to work.
After some diagnosis of their board I did the same calculations as you and came to the same conclusion.  15V, while within the limits of the regulator, overwhelmed the input of the op-amp.
I subsequently recommended a pair of clamping diodes on that input (now in Rev C of the Uno32), and since then there have been no reports of dead boards from too much input voltage.

(Note: I don't know if it's just a labelling problem in the PDF version of the schematic, but the VIN on the upper diode should actually be Vcc so it's clamped to the same supply voltage as the comparator.)