"100 Kts is the maximum speed that a single DASH actuation motor can compensate for cyclic stick giving enough negative stick to make necessary speed reduction should the need arises since one of the 2 DASH actuaion motors are inoperative?"
Not quite, it's more to do with the speed that the DASH can move, with both operating a single runaway is counteracted by the good DASH running in the opposite direction giving the pilot time to intervene. With one system failed this can't happen, and in a DASH retract at low level, it might not be recoverable at higher speeds.