No, it won't shorten the life, since there is little heat at the semiconductior junction- nothing like an incandescent bulb. Pulsing LEDs is the best way to get them to dim the most.
Yes you are correct that LEDs have a minimum forward voltage to allow current to flow across the junction. Its usually around 1.2v for red, and 3.6v for blue. FYI, We didn't have white until they figured out how to do blue.