I think that's the right way to play it - but even when their attitude hits Devoted, the Courtier doesn't own them. After all, mechanically, the NPC won't help out all that much (they aren't a Contact, so you can use Persuade for one action a scene, but that's normally it).
Owning them might be a bit of an overstatement, but don't forget Disposition is added as a bonus to several social checks, including Impress. A +25 on top of a Courtier's already impressive Impress check means the NPC better have an equally impressive Resolve check + the request is outrageous enough to give him a sufficiently large bonus on the resolve check (i.e. deadly risk and no incentive) or he'll do it. So yeah, a Courtier can pretty much boss around a Devoted NPC as much as he wants.
Also, there are no limits to how often you can Persuade someone per scene. You're probably thinking of Influence.