How straight could my back be after spinal fusion?

Depends. The outcome of a spinal fusion can be related to the re-alignment of the spine in what we consider sagittal and coronal planes. Spinal deformity surgery aims to maintain balance while correcting curvature and rotation to more "normal" parameters. Perfectly straight back alignment may not be as good as a well-balanced reconstruction. Speak with your surgeon about what to expect.
Depends. This depends on many factors type of scoliosis and how rigid the curve is the goal are to balance you head over your pelvis side to side and front to back.Safely.Get a solid fulsion and avoid progressive deformity.