I would like to talk about two things here: what do the players see from an adventure and why is there no need for a relationship map to have a good adventure.
First of all let's accept this axiom: "The purpose of all RPG play is learning." Wich leads to "this excludes as a valid purpose, namely aesthetics" (Mendel Schmiedekamp, PCon3).
Okay. Role-playing isn't about aesthetics. What does this mean? Lately I decided to introduce into the adventure a brother of one of the PCs whom about he never knew, but the scene was that obvious, that all the players knew at once, the he is the "lost brother". If we look at it as a story hook, it is rather lame, primitive etc. But the presence of this character made it for a better session. Why?
Because the players don't look at the story. They don't see it, they don't care. There is no need for a good written story. There is no need for good phrasing. These do not make a good adventure, these can only raise the enjoyment auxiliary.
Then what does make a good adventure? There are many answers, my answer is: player decisions. I think, that the only possibility for a player to take part in role-playing is to make decisions. If the player is passive, the adventure has to force the player to decide, and to decide about important things. I think the first part is easy to achieve, the second part is the funny. My opinion is that the GM has to use consequences to make a decision important.
On the other hand an adventure is similar to an aesthetical piece of fiction: it is heterogen, but it is one unit. Over at the Forge I read discussions about the difference between task and conflict resolution. I wouldn't like to go into it, but one of the questions was, wether the resolution is among characters or not. For example climbing a mountain isn't against the mountain in conflict resolution. I understand why, but the example is not perfect: what if the players are rivers and the NPCs can be mountains, too?
There is no real definition of what an NPC is. There is no real difference between an NPC and any condition that is actively present. On a long journey the horse can become an NPC. But there is no such thing as objective reality: you can't watch for lifesigns. Thatswhy the wagon can be an NPC, too. If you can write down that the nobleman NPC is getting sick on the journey, you can write that the horse NPC is slowing down on the 3rd day, you can write that the wagon has defect on the 4th day.
What do I want to say with that? There is no real possibility of a relationship map. And there is no need. Like a book or a movie, the players only see what they meet. If the GM writes a believable session, the players will believe it, anything might be the motivations for it. There is only one thing to account for: what is remembered by the players. Not what happened earlier, but what the players remember. It's an important difference. The GM's task is to repair these memories if needed.
My point is: there is no need for many-many NPCs, there is need only for one entity, the adventure. Don't bother with the NPC's intentions, bother with the adventure, what the PCs will experience, live through. If you got it, you will know the functions of the adventure (drama, fight, cast spell etc.). After you know the functions, you make up characters and/or happenings to carry these functions.
I think it is much simpler, and the players will have more fun, because there will be more things in the session that happen all the time. It's like action based adventure instead of NPC and/or world based adventure.