I had two back surgeries for herniated discs, at age 39 and 42. I was diligent about fitness before and after surgery. I took a break from riding for a year after the second surgery, and pursued programs that improved body awareness and spine alignment and stability. I swam for cardio (hated it....! but anything to get back riding!). For me, a careful program of pilates was the trick. It improved my riding, core strength, and awareness of alignment. This is so crucial for preventing further injury! Teaching pilates, mostly to riders, became my second career! Since then I've competed at Grand Prix dressage.
Yours is obviously a more complicated situation, with scoliosis, but whatever your decision about surgery, recognize the need for an ongoing program of fitness.