I know other people have talked about the Harnquists occasional disruptive absences and the problems they cause so here are a few possible ways to help:
1. Try testing out the movement system on an unimportant NPC until the system works properly. Let Harnquist stay where he is, the moving NPC to can have something good like a quest or skill but keep our important friend easily accesible.
2. Have Harnquist leave a note of some kind such as "Out to lunch: Should be back in a few hours. If you need me I'll be at the tavern" perhaps use the suggested signpost system based off the pets if that will ever be implemented.
3. Have a way to bring him back or keep him from leaving. Perhaps right before he wants to leave, have him say somethin like "I'm feeling hungry for <some bread, a fish, an apple>, I think I'll head to the tavern in a bit." make it a general quest and the first person to give him him the required food gets XP and he doesn't have to leave his post.
3. Allow players to train skills, I know this has been suggested before in many forum posts and I have read several of them. If NPC trainers are going to move around then it will be much more difficult for players to train skills.