Is the only way to fully straighten the spine through surgery?

In general, yes. It sounds like you may have scoliosis? In an adult patient, the only way to correct a spinal deformity and maintain it is with surgery. That is not to say that all patients with scoliosis need surgery, many can have their symptoms relieved no operatively; but in terms of correcting the curve itself, surgery is required to do that.
Yes if one is. Skelatally mature. Bracing in a growing child with scoliosis may limit scoliosis progression.