Can you restore your backed up private key into an newly sync wallet instance, then use the importaddress SbcthaEHqTyKGQUugECgX4meRTEsLAWkE3 "watch address" rescan command in the debug console to import the address?  Sorry I'm only guessing, I've never done that before.  In theory the private key is everything, and all public addresses are derived from it.  I've done some searching because I want to learn more, but couldn't find much about it or other people losing their coins, so I don't know how serious it is.