1) sigma (for the default carFollowModel) controls random slow-downs. Vehicles drive slower than their preferred/optimum speed in any given situation. This leads to a "ragged" look of the speed curve even in free flow conditions. In contrast, speedDev controls the deviation of preferred speed. This set a fixed random speedFactor value for each vehicle so in the absense of sigma each vehicle would drive with a constant speed in free flow but each vehicle would do this at a different speed.
2) departure time is only affected directly if option --random-depart-offset is set. You may see some effects from sigma and speedDev due to departDelay-interactions at high density though. Vehilce type drawing should be affected though.
Just for clarity: are you comparing simulation runs with different values of --seed?
