Agro, you might be interested in some relevant analysis I did recently: Power conservation analysis of linear and PWM brightness regulation methods
I think I can answer your question 2: with this linear driver the current of your highest mode would not affect the efficiency of lower modes. So there’s no negative to going with the 6A option, lower modes should be effectively equivalent to the 3A option. I guess the different versions of the driver (3A, 6A,…) have different sense resistors? Sorry led4power if this is not correct.