Optic nerve damage reversible?

Depends. On time, severity, etc. Specifically depends if the neurons have died (not recoverable at this time since an extension of the central nervous system) or are just in "distress" and could recover when the inflammation or inciting factor is resolved. Typically, not reversible to normal.
Almost never. Sometimes you can get lucky, and some of the optic nerve dysfunction is due to swelling, and when the swelling resolves, the vision improves. Thats the theory with ischemic optic neuropathy. But even so, the improvement is marginal.