You don't get to say "I roll to kill the orc" and if you succeed on your attack that automatically means the orc dies just because that is how you phrased it.
Which RPG are you talking about?
In Burning Wheel a player can do what you say can't be done. It's also a possibility in Prince Valiant (though more often it would be used for unhorsing rather than killing). It's also a possibility in 4e if the Orc is statted as a minion. And whether the Orc is statted as a minion is not necessarily just the GM's decision. In my 4e GMing I've adjudicated skill checks which have as their outcome the "minionisation" of a NPC. And I'd be surprised if I'm unique in that respect.
The orc has been introduced, and therefore you can try to kill it with your attack.
This doesn't seem relevant to the question I asked. There is no real Orc. There is no real attack. There are words spoken and dice rolled and numbers tracked and more words spoken. How is it not
authorship?
Your ability to find your brother through a check is going to be limited by the reality of where he is or is not. Again, like the orc, you are loading the outcome into your check.
This goes back to the question of
what "reality" of where he is or not? Given that you reject the notion that such "reality" might be established as the outcome of action resolution, I assume you mean
what the GM has written down or
what the GM has decided or maybe
what the GM determines by a roll on the random NPC location table.
Those all seem to be processes of establishing a fiction (which, in this context, = authorship). In the case of the brother, you want it to be authored by the GM unilaterally rather than via an action resolution process. Whereas in the case of the Orc you are happy to go the other way (but there's no reason in principle why the GM couldn't just decide that the Orc parries the attack, or that the GM couldn't roll on a
random NPC defensive manoeuvre table and get the result that the Orc parries the blow and disarms the PC).
Nothing you are pointing to about your preferences for different processes of authorship explains why
the player's ability to bring it about, via game play, that the fiction contains a dead Orc is not a process of authorship. What else would it be?