The character's backstory and how it related to the motivations is good stuff. I definitely encourage that and as a GM that sort of thing will most definitely inform my decision making, albeit not in some 'if this, then always this' fashion. What renders this cool and well thought-out player authored motivation significantly less meaningful, is that the player has the ability to author solution to their quest any moment they want. And sure, they need to roll dice and may fail, but that's still ultimately what's happening here.
Going to use this post to discuss Framing and Consequences and try to put together a post that will help you understand why the bold word here is a category error and why there was no violation of The Czege Principle.
The word you should be using is
propose. Author means fiat. You're stipulating a thing without resistance or recourse to dispute it. That is NOT what is happening in this case. The player is making a proposition and we're going to the dice to find out if (a) that proposition turns out to be a solution to his problem or (b) something else.
In one of my recent Blades games I had a similar situation to
@Ovinomancer so I'm going to lay out the gist of it and show you how
propose does not equal author.
Here though is the formula for agency:
Dictate what the game is about =
player agency
Specific proposal + GMing ethos to follow their lead and play to find out =
player agency
All the player-facing tech =
player agency
Engagement Roll procedures =
player agency
The deep suite of resources that the players can bring to bear to turn this
proposition into reality =
player agency
Now just to be clear (as you're about to see below)
all of that player agency does not equal success.
Tier 2 Crew w/ a Whisper.
In order to amplify his power and to get them out of a huge predicament, the Whisper made a bargain with a powerful poltergeist (a member of the Reconciled, Magnitude 3, so 1 higher than the Crew). Unfortunately, the possession (its not constant control because of the Whisper's resistance to the supernatural, but it manifests and in not-great ways at not-great times) is wrecking the Crew's life and slowly taking the Whisper (every week of possession you take Trauma...4 Trauma and you're toast). The Whisper had a contingency sorted out when he made the deal; a Longterm Project to perform a self-exorcism. Unfortunately, the Downtime Project rolls just aren't going well and the Clock isn't filling fast enough.
So the Crew decides that they're going to go with a Linked Plan (a Split-Score):
STEALTH - Break into the Duskvol Academy's basement (where the Whisper went to school) where a Reconciled (a Tier 3 group of ancient spirits that don't lose their faculties to the ravages of time) Spirit Well is secretly located (the recon for this was performed during Information Gathering/Free Play), secure the powerful arcane energy (that sustains the Reconciled) from the well, then...
OCCULT - Exorcise the Whisper once they have the powerful arcane energy from the Spirit Well.
During the Stealth portion of the mission, the Crew is in the Basement. While everyone is is doing the heavy work of excavating the Spirit Well, the Whisper is perusing the forbidden occult books and artifacts in the Academy's basement. He's looking for a Ouija Board to help him uncover the Truename of the spirit for the coming exorcism. Of course he finds it. This should help him in the upcoming exorcism; giving them an extra die on the Engagement roll for the Occult Score and +1 Effect the first time he uses it in the Score (which is going to be a Tug-of-War Clock...if the PCs win, the exorcism is complete...if the Spirit wins, the possession persists and the PC takes another Trauma).
The Ouija Board is a powerful conduit to the Ghost Field, so interacting with it is extremely dangerous. However, the Whisper has his Spiritbane Charm in his mission Loadout so it helps ward him a bit. We're doing Racing Clocks here and starting with Controlled Position due to the Charm. If the Whisper fills his Clock first, he gets the benefits above. If the Ouija Board Clock fills first, then the Reconciled Spirit becomes aware of his attempt to find its Truename and will be enraged and bulwarked against the exorcism; -1 die to the Engagement Roll. This will happen along any Stress that is incurred along the way of the interaction.
So, I won't go over every Action Roll here, but the gist of it was this:
1) The Whisper attained a Pyrrhic Victory with the Ouija Board. Yes, it won its Racing Clock battle and attained the Truename of the Reconciled (thus giving it the benefits depicted above and avoiding the -1 Engagement Roll), but it incurred enough Stress in 2 Resistance Rolls to put him in a really bad spot for the upcoming exorcism.
2) Due to all the Stress that the Whisper was dealing with (if you incur enough Stress you (a) incur Trauma and (b) are knocked out of the scene) now because of that Pyrrhic Victory, he only had 2 Stress Boxes left. So he couldn't Push himself or use the elaborate Flashback he had planned for the exorcism. Ultimately, this very likely ended up being the difference as they lost the Tug-of-War Clock for the Occult-exorcism phase of the Linked Scores.
Because of that, the Whisper earned another Trauma and then later earned the final level of Trauma because the Downtime Project Clock couldn't clear the possession in time.
So the Whisper and the Crew has all kinds of agency to all 3 of (a) dictate what the fiction was about (on both the macro and the micro), (b) propose changes to the fiction, (c) draw upon their significant resources to make that proposal actionable.
They just ultimately failed because that is what happens (as everyone knows) in games. Failure is always on the table.