MAB, aside from being a phenomenal player, is a great showman. I’d bet the ranch that the reasons he does the overhand fretting at times is not technical at all and he just thinks it looks cool and flashy. Maybe an edge case would be if there are very wide stretches, because I think that’s easier since it allows the wrist to be in a more advantageous position, like a piano player. And I think I have seen him do the overhand thing with wide stretches.
But to your point about the ordering of fingers used, my classical teacher always told me that it was ‘easier’ to go from weaker fingers towards the stronger. He said that’s why classical tremolo (ignoring the thumb) is a-m-i and not i-m-a. So from this it would follow that if the pinky was included, it’s easier to do c-a-m-i as opposed to i-m-a-c. Certainly, if I just sit at a table and tap my fingers I have an easier time going faster when I start with the pinky.
@Tom_Gilroy has done extensive research on left hand speed and concluded that combinations involving ring/pinky are always slower that index/middle/ring or index/middle/pinky. As far as I understood, in terms of speed potential there was no difference in a forward or reverse pattern as long each finger always was allowed to rest the period of the 2 other fingers being used.
I’ve never heard him comment on the scenario of, if one must do ring/pinky in succession, if there is a natural preference for ascending or descending.