you got two options at least to remedy the bag rubbing on the derailler.
1) install one of them wire-cage derailler protector thingies that mount at the dropout.
2) switch to a rack designed for bikes with disc brakes. mine moves the rear of the pannier a full 4cm from the chainstay, just enough to clear the derailler in the fully out position.
assuming some flex from the pannier might cause rubbing in bumpy terrain, should be easy enough to fashion a small plastic protector to mount on the rack to keep the bottom of the bag from hitting. prolly not needed as i rarely (~never) use the wee, tiny cogs anyways.