Unseen Servant is nice, but is a 20 lb limit spell that you have a free 5 lb version of for the most part. It can do more complicated things, but the range the servant needs to stay in without disappearing is the drawback. Riding Lu, you can easily outpace the servant and have it disappear due to range. Even Ni, double moving quite a bit could do the same. So, it may not last as long as you hope in the idle state.
Spell Suggestions:
Shield: always a good spell when you have prep time to fire it off.
Get a Lesser Rod of Reach for 3k and you won't have to be within touching distance to slap it on Lu.
Ray of Sickening was a good suggestion. same reasons.
Expeditious Retreat: Lu moving along at 70'.
Best when paired with Unfetter. But, since you can ride Lu, the pairing of spells is not as critical.
Compel Hostility looked nice until I figured out that they have to already be within threatening range, so it cannot be used to force movement.
Protection from Evil is something you want on a wand. When you need it, you want to cast it five or six times in a row to cover the whole party. Summoner's could do that, but then exhaust their spell reservoir severely.
Unseen Servant is better as a wand spell. Well, it is hard to think of what task you need to command it with that will take more than an hour. So, the extra duration of knowing it yourself seems moot.