Cooler weather outside makes for more dense air. More dense air requires more fuel to keep the air/fuel mix correct. So, you would want to richen things up a smidge.
As for the outside air being cooler and causing lower running temps, that is possible. You will get better performance from the cooling head with cooler outside air. The engine will in turn perform a little better due to the fact that the engine works on a combustion cycle that uses exhaust gas and the expansion of said gas to move the piston. The bigger the temperature split between the intake air and the combustion temp, the more rapid the expansion and the more energy created within the chamber. This in turn means more power from the engine.
So...yes, the engine should run cooler on cooler days due to better cooling from the engine's cooling head. Yes, you need to adjust your needles to get the same performance. No, the cooler temps shouldn't impact engine life, unless they are way too cool.
Hope this helps.
