How much growth would you expect the fused portion of my spine to have grown had it been left unfused?

That is dependent on. Your age at the time of surgery, the level of the surgery and the number of vertebrae fused.
Hard to say. It depends on how many were fused and at what age. If not fuse and you did proper exercises your cartillage would be elastic and could stretch giving you extra height. With a fusion you can not do this stretch.