@Manbearcat
This went super long, if you don't have the time to read it that's ok. I find writing this type of stuff out helps me with y own clarity of thought.
In what follows I'm going to talk about Narrativism and disregard Gamism entirely. I'm going to (hopefully) answer your questions but in some cases it's easier to fully explain a concept than answer directly because there are some points where you've misunderstood me.
I still think we have some fundamental disagreements, if my understanding of what you've written is correct. I need to go over some stuff before I address them directly.
WHAT IS FIAT?
First up is the drift (and I'll take responsibility for this) in meaning of the term fiat. I've ended up using it in two, kind of three, different ways.
1) making a call based on the fiction
2) authority over a certain part of (backstory, resolution, situation, outcome)
3) authority over resolution
When I say fiat is at the heart of roleplay I mean it in the sense of 2. If there is a meaningful distinction between fiat and authority then I'll retract my statement. As it pertains to 3, the only way the GM can have total authority over resolution is if they 'also' state how your character reacts to something an NPC said. The GM can have outcome authority over PC actions towards physical activity and how NPC's react and a Narrativist agenda can still be fulfilled. Whether that's 'good' is a different matter, it's just not destructive to the agenda.
AGENDA AND SITUATION
I'll need to lay out a situation we both understand to get any kind of practical communication happening. So hopefully you've seen Return of the Jedi and I'll be putting it into a role-playing context.
What's the situation? I'm going to assume that last session the players killed Jabba the Hut. I go away and prep the next situation by taking some elements that already exist and creating a whole load of my own.
I want to see what happens between Darth and Luke, between Han and Leia and between the Rebellion and the Empire.
I come up with the idea that the Emperor has leaked his location to the rebellion as part of a plan to lure them in and destroy them. He's going to have a new Death Star and it's going to be fully operational, he's going to have a squadron of his best storm troopers down on The Forest Moon, he's going to have the whole Imperial fleet lying in wait.
I prep the situation by creating the various elements in play:
Luke Skywalker, Darth Vader, The Emperor, a false squadron of storm troopers, a shield generator, an elite full battalion of storm troopers, Ewoks, a ship with old imperial passcodes, the rebel fleet, admiral Ackbar, Lando Calrrision, the millenium falcon, Leia, Han solo, Chewie, a partially complete but operational death star, the entire imperial fleet.
I think those are all the elements and then their is positioning between them.
(a very partial list) Darth wants Luke to join the Dark side, Han and Leia have a kind of thing together maybe, The Ewoks want to eat people. So the ethos and backstory of the various characters in place.
More physical positioning, a partial list. (the shield generator protects the death star, the death stars shield is up, Darth Vader is on the Super star destroyer)
So I've got my situation and situation is in a state of either being Fully Known or Partially Known.
If I show this prep to the players, including the plans of the Emperor, the back story. Then it's Fully Known to everyone at the table. If I keep the prep secret, then it's Fully Known to me and Partially known to the players.
Next it has to be mutually aesthetically parsed for it to mean anything. As a group we need to understand that there is a thing between Luke and Leia and we want to know how it goes. As a group we need to understand that Darth is trying to recruit Luke and as a group we need to understand that and want to know how it goes.
Another way of putting this is that we have questions about the situation and we're excited to find out about them.
Situation and not protagonist. We're excited about The Situation, we're playing to see how the situation resolves. Imagine that we've all seen my prep so the situation is Fully Known. Then we could choose to play through it with different systems taking on different characters. I might decide I don't want to GM and instead choose to play Darth Vader for instance. This is an extreme example but it's showing the primacy of situation as it relates to our mutual agenda.
THE JOB OF THE GM
So I'm assuming a system like 5E or Sorcerer. The standard division of labour. The GM has two primary jobs.
ONE: Play the NPC's as if they were real people* (a more technical version later)
TWO: Frame scenes based on a mixture of fictional positioning and what they want to see next (although the players kind of have this power as well, the GM has final say).
If the players say they all go to the Forest Moon on the old Imperial shuttle, then the next scene might be them arriving, them at a checkpoint while the codes are cleared, them in a docking bay before they leave, maybe the GM frames a scene with just Han and Leia whilst everyone is getting ready to go.
Framing scenes also means deciding the scene specific elements AND fictional positioning at some level of granularity.
The GM frames a scene where the shuttle is being asked to give codes. The elements of the scene are: Darth Vader (off screen), Imperial ships, the shuttle, Luke, Han, Leia, Chewie, the old Imperial ship, the codes, the guy aboard the super star destroyer who clears the codes.
THE PLAY OF THE SCENE BEGINS
I describe the Imperial shuttle and what they see outside. The star destroyer, the forest moon, I say the imperial officer is asking for codes AND
Let's get a bit interesting. I'm thinking off screen about Darth Vader and what his powers are and his connection to Luke. I'm thinking about whether the codes are old and that's going to cause problems. I decide that Vader can probably sense them given his fore powers and relationship with Luke. I ask the players what they're doing and tell Luke he can sense Vader maybe sensing him.
If this is Sorcerer we 'have' to roll if it's a conflict and so we need to know if it. I ask Luke's player if he wants to avoid detection and Luke's player decides he does. We make a conflict roll and Luke fails. I narrate that he knows Vader knows he's there. Luke says 'I shouldn't have come, I'm endangering the mission.'
THE SITUATION HAS CHANGED
I think about how what just happened has effected the fictional situational. I decide that Darth Vader would, given what I know about him, go and speak to the Emperor and the Emperor would tell him to go down to the Forest Moon.
FICTIONAL CONSTRAINTS AND THE BASIC UNIT OF PLAY
Upon learning that Luke is onboard the shuttle. I decide that Darth Vader goes and tells the Emperor.
How did I decide that? Well how it feels is that I'm inhabiting Darth Vader and this is just what Darth Vader would do and so I make that choice in response to the fiction and my understanding of the fiction.
But what's also happening is that I'm seeing that Vader IS going to go and ask the Emperor rather than anything else he may do. Which is an expression of his current ethos as it stands in respect to his relationship with Luke.
So the relationship between how it feels 'this is what would happen' and my expression as an artist is related. I'm using the medium, fictional positioning within a situation and the choices I make within that medium are expressive choices.
ACTIONABLE DECISIONS AND GAME STATE
You wrote:
I have more to say on the matter that is related to my extreme skepticism over Sim-Immersionism's ability to either (1) generate a gameable decision-scape or (2) consistently center PCs as protagonists. My skepticism around these (2) is because GM-based setting extrapolations inexorably put extreme pressure upon play to be "real/legitimate" based upon the GM's, imo extremely fallible, causality-based extrapolations + sense of continuity and that formulation has a tendency to "backseat PC protagonism"
It seems we have major points of contention, maybe. What is PC protagonism? I want to restate what I think you're saying in my own words to make sure we're on the same page.
As a group we're invested in how the situation plays out but there are two factors that give a kind of determinism to it.
Do the characters know about element and backstory factors that are determinative of certain outcomes within the situation?
Let's say they blow the shield generator. How do I decide what happens next? Let's say I haven't prepared the result of a potential action. My extrapolation of the given factors is based on what would happen. What I decide that despite the shield generator being blown. The Imperial fleet + death star wipe out most of the rebel fleet anyway?
My reading of what you're saying is that this would be terrible? The PC's don't know it's going to happen so have no choice in whether it does. It also renders the attack on the shield generator worthless.
I think the choice of the GM to do that is fine. It's what they think would happen and so it's an expression of ethos.
And this is where it gets a bit tricky. Although I think it's fine I also think that the more information that is out there the better. All else being equal I think it's best to telegraph certain consequences but 'why' I think that and why you think that (If I understand you) are totally different.
Am I correct in thinking. You want the choice to be telegraphed and known because the PLAYER is invested in the outcome.
I want it telegraphed because it gives the CHARACTER more choices and thus more chances to express ethos by 'doing what the character would do.' WITHIN the context of the situation and how it plays out.
This may be a fundamental disagreement if I'm right.
CLOSING
To really hammer the point home.
I do think that the GM 'doing what they think would happen,' Does or can give legible choices vis a vis the positioned fiction BUT I don't care (much). This is primarily a gamist concern.
To quote myself from earlier:
You extrapolate based on (1) their priorities/personality (2) what's actually available in the setting which can be fairly broad (3) the resources, positioning of the person doing the hiring.
Jackson has deep roots to the criminal underworld and loyalty matters to him. He's precise, a good judge of character, not massively wealthy. His assassin is someone who is genuinely loyal, deadly, patient.
Bellow is rich, dumb and impressed by trinkets. His assassin is good because of the wealth but also a braggadocio and not as deadly, loyal or patient as Jacksons assassin.
A different player, given the same fictional material to work with, would make a different type of extrapolation. The type of extrapolation made, the causal connection. That's the GM addressing premise. This type of person gets this type of assassin.
This one of the fundamental interactions that allows artistic expression using the medium. It IS within the context of off-screen action but that off-screen action becomes on-screen action.
When it's turned to on-screen action. Characters within a scene, interacting:
I've quoted this elsewhere before.
You know that thing where you're so into your character that you adopt her emotions, mannerisms, outlook, mood, heart and soul? It's a rush? You aren't thinking about your character, you just do what she'd do without thinking? It gives you deep insights into your character that turn out, on reflection, to be deep insights into yourself, your friends, and the world? It feels totally alien and natural at once? You crave it? That's what I mean by immersion. I assume that's what everybody means by it.
If it's an NPC talking to a PC and the player can choose how their character reacts. Then the player has outcome authority.
(and yeah you can add dice to that and I usually do but it isn't necessary because the outcome authority, even in a lot of trad games, is distributed in this precise way)
Sorry for the length. One last quick thing:
The legitimacy I talked about in the alarm example was solely because the scene was 'in situ'. Adding elements to the scene which involve the creation of backstory to explain them is kind of poor form in my opinion.