[comdpa]

So, is it possible to have too much shoulder turn on the backswing, even with a good pivot?

The purpose of the shoulder turn on the backstroke is to position the right shoulder on plane.

Therefore, if one is swinging on a Turning Shoulder Plane ala Colin Montgomerie and turns his shoulders to a degree more suitable for a Turned Shoulder Plane, then yes, it is possible to have too much turn on the backstroke.