In terms of ease of math, the move to adding the bonuses, rather than averaging them, was an improvement in RMSS. There was some inflation in averaging them up.
I beleive that would be fixed if you smoothed the bonuses first. I always prefered the smoothed bonus in RM2 (i.e. . . .-3,-2,-1,0,+1,+2,+3,+4,+5. . . rather than . . .-5,0,+5,+10. . .)
Which kind of answers two of the other questions you asked.