So some of the examples of the players changing the fiction has been "My players did something I didn't expect and an encounter or campaign arc didn't go as I thought it would."
That's not changing the fiction IMHO. Changing the fiction would be the players deciding that the BBEG is not really a homicidal maniac who just wants to see the world burn is instead just misunderstood and with the right approach is actually a nice guy. It's the players deciding that they find a secret passage not created by the DM that lets them escape or bypass a significant threat. It's a player that says, without DM approval or input, that they have a map showing the layout of the castle.
Players overriding the fiction, to me, is players changing the world and external circumstances because the player says so and not by the actions of the PCs. If the PCs can take out your BBEG in 1 round or cleverly trap your monster, good on them! They got lucky or were clever and the PCs actions changed what the DM had expected is not overriding the fiction, it's helping to build a narrative.
I agree. A DM that gets surprised and has something wacky happen because of that, does not fit changing fiction. That is just a DM problem.
Though the PCs digging a new tunnel has nothing to do with "the fiction". And a player randomly creating things is just cheating.
The fiction overrode me as DM. I wanted to have that (hopefully) awesome fight. The fiction said that didn't make sense, so it didn't happen. My players were actually very happy, not disappointed, because they appreciated that I had enough respect for their actions to not force things to happen simply because I thought it would be cool.
That is a nice story, but it does not fit the question. Everything still happened the way you wanted it too. You chose to toss the monster away and chose to give it some super water weakness.
Of course, your long post is a lot of set up, but then has no details of that final fight. From what you typed it sounds like the players came up with the wacky idea, and you just said "it works". Though they had to roll to destroy the monster? Why did the "fiction" say the monster would just auto walk into the trap?
It's a lot more like you had a "Linear Plot Idea"(aka the "R" thing that shall not be mentioned), then when your players had a wacky idea, you suddenly changed it.
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If we count "The Fiction" as everything except the game rules and mechanics, that is a good start. And that everything is all on the DM (with yes a two copper input from some players sometimes). So the DM can make, break, change, or do whatever they want to the Everything Fiction.
A DM can say "no elves are allowed to join the Dark Dreamers group", and then have an elf member of that group show up (with some unique history of how she joined).
The DM can say "all members of the Order of Holy Light must destroy all undead on sight", and then have members let a good undead PC go.
There really can't be any time where the DM has set some fiction, then later decides to change something, that it's not ALL the DMs doing. Sure, the DM can say they changed things because they "thought it fit the story better" or "liked their new idea better", but that is still just the DM changing things.
The DM can't change things "against their will" : they have to agree to change things for them to happen. And if, somehow, a DM was backed in a corner.......tht DM can STILL do whatever they want.