1) Can a person need to go in a certain order of steps [in Avodas Hashem] or is it possible for a person to skip and go straight to the desired level?
2) Did the tzaddikim throughout the generations reach the levels which the Rav describes?
1) By way of mesirus nefesh (self-sacrifice) a person can skip levels. This is only for individuals.
2) Each one reached it, according to his level and according to his soul root.