Righteous fire is only supposed to affect spell casting. They'll still be able to attack or use abilities if they have them. So the NPC would be barred from casting Heal or Blizzard, but could still use Water Strike. Given the choice, I'd rather have one of my front row slaves take a little damage than having my back row get stunned via blizzard. I haven't done much testing to see what qualifies as a "spell" with higher level critters in the ancient jungle or molten core.
Loading saves with Import Mode toggled on should let you keep all your characters, but I haven't messed around with it. I just start a new game when versions change.