I have found that the key to managing the early approach is the first step; the fault lies in not only being too early but too large. Of course, as a coach you can say 'wait' but then the MB's still take that big first step which commits them to an early approach. Some Old School coaches actually do the hold the shirt of the MB to slow them down!
What I do with early approaching Middles is get them to drastically shorten their first step down to where it is almost a stutter step length. In a sense, it is a half step, where their heel may not touch the ground and this shorter stride will slow them down. On a 31 approach, there should only be 3 steps in the approach; first step is a lateral half step (lateral with the 3 meter line) and then the next two are explosive steps, as the Middle turns the hips and shoulders toward the setter. This half step and then explode should help with not being inverted on her approach.
With the 31 attack set, I prefer my Middles to be just coming up from the bottom of their squat jump when the ball is coming out of the setters hands. This gives them a bit of a window to adjust and gives the setter a little more timing cushion to deliver the ball.
The best way to practice the MB stutter and explode footwork is hundred of reps with small ball/tennis balls, where they half step, hesitate, then explode and throw the ball over the net. Outside hitters can be a bit more free flowing and organic in their approach movements, but middle hitters must be concise and this exactness is developed through footwork routines.
Hope that helps a bit.