It's organic. If they say something that forces me to respond in a way that something that wasn't there before is now, they've caused it to be in the game.
I mean, you can think the Earth revolves around the moon for all that I care, and you'd be just as accurate!
Everything you described there is a DM being adversarial. Outright saying no via tampering, the perpetrator knowing, etc. is the DM countering the player declarations without cause, which is both adversarial and bad faith DMing.
No, it's not! How do you figure? If they say "what about the cameras" and the DM hadn't thought of that, then that's it? Mystery solved. They watch the video and there's Colonel Mustard with the rope, plain as day!
Of course not.
So the players prompt the DM with an unexpected question... what about the cameras... and the DM then has to determine what the cameras may reveal. Likely a clue of some sort, no? Okay... what kind of clue. How useful a clue? Is it something concrete like a monogrammed scarf? Or something that implies some new information... like an altered video would indicate someone inside being involved?
How is any of this determined?
The DM could make it some sort of skill check and on a failure those things might happen, but outside of corner cases, the answer either needs to be yes or reasonable chance of success via a roll.
Right! The DM "could" do that. He "could" do a lot of things.
And my point to you all along has been the more that all of this stuff is up to the DM... all the background information, the new information that needs to be introduced into play, how useful it is, what if any mechanics are used to determine all this, how difficult a task may be, and on and on and on.... that impacts the players' ability to influence play.
It doesn't remove it! As you say, the players can prompt the DM with questions to possibly reveal new stuff... of some unknown scope or importance or relevance, using some methods that the DM may decide at the time!
This is why I say I'll believe it when someone can offer a clear description of play that doesn't rely on vague stuff like "it's organic".
Objectivity the way we are using it doesn't need to be 100%. What we write down is objectively true for for the fictional mystery, but that doesn't mean that everything possible has been written down.
That's a useless definition of DM fiat. It broadens to the point where it confuses things greatly. A DM making a decision within the rules of the game is not using fiat. He's using rules. To be fiat you need to step outside of the rules.
Forget the term DM fiat for a minute and just look at what I'm saying.
DM input on all this stuff... it's so much DM input that it's hard to track it all.
I think this happens and people stop attributing these things to DM choice at all... they start to say things like 'well that's what would most likely happen' and so on. But it's only what would most likely happen because of all the contributing factors... which are mostly made by the DM.
And adding their ideas. If I didn't think of it and it gets added to the game it wasn't my idea that came into play. Yes, their ability to bring ideas into the game is much more limited than a player facing game, but its not 100% DM like you keep implying. I can be forced by the social contract to act in good faith and add what they thought of into the game.
What ideas of the players get added to play?
The existence of cameras.
Okay... then what? What do the cameras reveal? Whose idea is that?