If you killed Sua, then you cannot summon him and the white witch cannot be freed.

Go back to a save before you killed Lord Sua and then you can finish the quest. Also, don't do anything dumb like this in the game, it is quite easy to break quests. It is a consequence for killing important NPCs.

Every time there I run into trouble on the road, there is always a dwarf at the bottom of it. Don't they know how to drive above ground?