Thanks for the compliments on our driver and the great feedback! It would be convenient for all of the external connections to the shield and controller to be on one side and for the remapping jumpers to be closer to extra I/O pins. Unfortunately, the USB connector and barrel jack on the Arduino Uno would interfere with the leads of the terminal blocks that stick out the bottom of their through holes if we were to just move them to the opposite edge of the board. As for the pin mapping, we tried to pick the least useful Arduino pins that could work with our shield. It's hard to avoid incompatible pin choices with so many different shields people might want to combine. We will keep all of your feedback in mind for any future revisions or shields.
We do not have a particular maximum operating temperature rating for these regulators. In general, as the temperature goes up, the lifetime of various components goes down. Among the components likely to limit lifetime are the larger capacitors, which have a rating of 2000 hours at 105 degrees C. However, other components typically get much hotter than the capacitor, and the temperature distribution across the board depends on many factors, such as ambient temperature, load current, and input voltage.
If you are seeing only 50 degrees (how are you measuring that?) with 12 V in, 14 V should be fine. How much current are you drawing?
The mounting holes on these regulators (and most of our other boards) are not electrically connected to ground or any other node in the circuit. If you want to mount the regulator to a heat sink with maximum effectiveness, you should probably use some thermal gap-filling material so that the side with the low-profile components can conduct heat into your heat sink. Be careful not to short anything out on the heat sink!
Sorry, we don't have those characteristics well characterized since they vary a lot with parameters like load and input voltage. We do not especially optimize for ripple, but it will generally be under a few hundred millivolts peak to peak. The switching frequency changes with load, but the highest frequency is around 600 kHz.
In your third paragraph it is not clear to me what "adverse consequences" and "broader community" you are referring to. Could you be more specific about what consequences and community you are talking about?