The first part, seems like a classic miscommunication problem which can be solved by just talking about it after it first appear but before it becomes a major issue. It is all about expectation and disclosure of what the GM and player want. The first part of your question would superbly server as an example of what you are trying to do. Making sure that the players understood that might have averted the whole situation.

That said, your player could have mentioned her disappointment better. Being angry of miffed is hardly the way to go about it. Being assertive on the other hand, always is a good thing.

To avoid most of those, I tend to use the (American) football time out sign. Once that's out, we are no longer in play and more in meta game. A quick explanation, re-wind, and play generally does wonder to solve those tiny annoyances that will grow into big arguments if left unchecked.

As to your second point... I tend to know the kernel of my world, of how things work, of the NPCs motivations and information. From there, I improvise. I may have 60 NPCs (names, a profession, an image, whatnot) and as the PCs decides to interact with them, I fill their backgrounds up with interesting details. The world is as detailed as the PCs (and myself) need it to be and no more. The rest is cardboard cut ups. Generally for a game running 15 to 30 sessions, I will have about 10 to 20 pages of notes at the starts and about four times that by the time we are done.

The first part, seems like a classic miscommunication problem which can be solved by just talking about it after it first appears but before it becomes a major issue. It is all about expectations and disclosure of what the GM and player want. The first part of your question would serve superbly as an example of what you are trying to do. Making sure the players understood that might have averted the whole situation.

That said, your player could have mentioned her disappointment better. Being angry or miffed is hardly the way to go about it. Being assertive on the other hand, is always a good thing.

To avoid most of those, I tend to use the (American) football time out sign. Once that's out, we are no longer in play and more in meta game. A quick explanation, re-wind, and play generally does wonders to solve those tiny annoyances that will grow into big arguments if left unchecked.

As to your second point... I tend to know the kernel of my world, of how things work, of the NPCs' motivations and information. From there, I improvise. I may have 60 NPCs (names, a profession, an image, whatnot) and as the PCs decide to interact with them, I fill up their backgrounds with interesting details. The world is as detailed as the PCs (and myself) need it to be and no more. The rest is cardboard cut outs. Generally for a game running 15 to 30 sessions, I will have about 10 to 20 pages of notes at the start and about four times that by the time we are done.