I'd persevere myself. a2p described the steps involved very well. Providing you get the hi/lo stops set right the main solution for solving the shift problem will be in the cable tension. Once you know which stop screw does what take some tools on the road with you and do a series of tweaks as you go along. The cable adjustment is the simplest to do.

You mentioned you had a rear der hanger bent back into shape. From where I sit, it should have been replaced - assuming it's a bolt on? Could it possibly be out of alignment ever so slightly, to the point that the rear der is running askew?