I can totally agree with first point.
In my current saga I rejoined a group I left 8 months earlier due to motivation losses for the game itself. The players had restarted a saga because of new players joining at the moment I left.
I rejoined, and soon after 2 sessions, was asked to be beta story guide again, as I was almost from the moment I started the game (first AM campaign game in august 2008, first time beta SG in october 2008). I did a simple themed adventure and invented NPCs on the fly.
For my second scenario, the alpha story guide gave ma a piece of a (available online) saga about 12 ivory coins. The part was about a piece in Arcadia. I had nothing against this scenario but I dislike the way the author went into his scenario, so I personally modified almost everything. I decided to include a disney theme - Frozen - and Season Court of Winter in Arcadia. (From that moment, Arcadia CourtS of Season become the place for me to drop Disney names : I felt that so inspiring, and players react well, because a name immediatly give a feeling of personality for any NPC. If I say the summer king is Jafar, they immediatly understand ; if instead I say it's Mowgli, dang, immediate differenciation spotted!).
Then 1 month later, I knew I would have to take another adventure, and I decided "okay, end to those one shots experiences", and I tied all of this together.
I used a covenant we used in a previous saga 5 years sooner. I used the NPCs, created relations between them, and a purpose. The purpose backfired. I linked it to my first adventure and some of its NPCs,n and to the second.
It ended with a 4 pages "history fact sheet".
Now, that history fact sheet is 25 pages long, covers almost 3 millenia, includes almost a hundred of NPCs and I had to use a relationship chart website to include them inside, because after having 200+ NPCs, I started to lose track of their internal relation to themselves and PCs.
If i start writing about a NPC, i check the history fact sheet, I check the relationship chart to know who he is, what he does.
It allows me to be very spontaneous and never lose my way, even when the players decide they are not into the (prepared) adventure, but in some of their personal projects.
I know my NPCs, I know what they want and why they want it. I know which NPC is spying on who, etc.
For example: the PCs have in their covenant a NPC from almost 20 years now (+-1,5 years IRL) and they know that person is very important. They protect the person, but only bit by bit do I give information to the PC friend, because the NPC didn't say all of the story in one go. The NPC had to build trust, and even trusting, she kept things for herself because of shame, because she thought it was unimportant, because she didn't remember it clearly before.
Sometimes, the NPC will contradict itself, because events happened 50 years ago to he and he forget or make up a fiction tale to encompass what he remembers.
I have felt the players do appreciate NPC who are not just pieces of scenario, but characters seeming authentic.
As for the story, they have an idea of what is coming (thanks twilight and arcadia, premonition and angels), they know there are factions in the world, and they navigate between them.
A scenario is either a part of the grand scheme, either a moment to give away some information under cover. Most of the times, the players will learn something for the immediate situation, but underneath it, another information which, added to another previous informaiton, give them a better comprehension of the grand scheme.
Myself for the sagas of the 2 other SGs, I write the summaries in a google drive file, to not forget. Other players do this for my setting.
If players do nothing (and they have for most of 7 last years now) the "antagonists" (if I may call them that way since they have objectives of their own, which are more or less profitable to the PCs) still advance. I have milestones in my history sheet fact. If players remain passive, the NPC will win without any chance for the players to change it, even if they "succeed" in the next scenarios, because a war is not only won by reacting. Since this is a roleplaying game, and not a "react to story game", I expect players to take actions based on the (obvious or not) clues I give, together or by themselves.
The story is a complex thing, and they are in. If they let themselves carried by its flow, they will eventually fail, and as my HR-manager said once "it's okay too", because that makes a good story too.