Nomic possibility isn't supersalient, I don't think, because it's rare that a fictional event admits of only one nomic explanation within the context of the overarching fiction. I've just flagged an alternative possibility, to that of a twin sister, to explain eating two meals. And as I already posted upthread I think there can be alternative nomic explanations of why a knife wound doesn't bleed - like that the victim had no blood for some other reason (eg some sort of unnatural spirit being, or an alien morphed into human form, or something else that fits the genre - relatively few RPGs seems to be run in super-realistic genres where no fantastic/supernatural explanation would be possible).
I mean, yes, those are
theoretically valid alternative explanations....but not
actually valid in context. The victim was an earth genie, and not a noble one (he had a position within the royal court, but was not ascended as is required for being "noble" in this setting). Meaning, he had a body which functions exactly like any other body.
Like...these things you mention are exactly the "there's a justification why this isn't the case" things that I was talking about. Either I have to keep the thing as it is, or I have to give it a real, meaningful justification for why it's not. I can't just
decide "nope, different perp, just 'cause I feel like it." Even if the reason I feel like it is because the players have come up with a really really cool theory, I have to actually do the work of
justifying that theory. Otherwise, I'm pulling the wool over my players' eyes; I'm making them
believe that there is a real, durable, understandable, predictable world, when there isn't one, there's just "stuff I said before" and "stuff I'm saying now," and the stuff I said before is literally 100% completely malleable if it sounds better for what I'm saying now.
What is salient is what reason, if any, does a GM have (i) to do prep, and (ii) to hold onto it? These are extremely important questions from the point of view of approaching RPG, and different ways of answering them will produce very different RPG experiences (such as, to reiterate, the difference between spending a couple of relaxed hours solving a whodunnit, and getting caught up in the emotionally intense play typical of BW or AW).
Well, for me, the answers, in order, are:
1. Because, by preparing, I have more to leverage. I can improvise, but I find that I only have a finite amount of improv in me before I have to tap out. This amount is
usually sufficient to get me through the vast majority of sessions, but there have been a few times where I've had to say, "Alright guys, we'll break a little early today, and I'll do some prep work for next week." Or something to that effect. A secondary reason is, well, that the DW rules
tell me to prep--one of my DM moves is "exploit your prep," after all. But I try to keep that prep work focused and relatively light, so that I can respond to what the players do, not make them jump through pre-established hoops.
2. I assume this means "adhere to" rather than "keep secret." My reason for adhering to prep is that I want my players to know that the world is (in some sense) real, durable, predictable. That they can always attempt to validly reason from the information they have. Any errors in that reasoning will
never be because I altered anything about the world, but because they were mistaken about something--a mistake they
could have avoided, it just happened to be the case that they did not.
This is why I use the murder-mystery example. If I switch up the killer
solely because the players had a good idea, then one of two things happened. The first, as previously discussed, is that I screwed up
really badly (and that isn't impossible! My players totally have outsmarted me, more than once!) The second is that I'm now changing what stuff is in the world, but in a way that the players should never be able to determine
even in principle, because while
last week they would have been wrong to accuse the Princess,
this week they're now correct, but nothing about the things they found has changed in any way. The
past events have changed, but
literally all the evidence available to them has not.
That's why, as I've said, I see "my players have found a theory which is straight-up superior to mine, and which uses the exact same evidence to point to a different perpetrator" as such a massive failure on my part. It means that I have provided evidence that was
supposed to enable clear, rational,
reliable reasoning to determine the truth. Instead, not only did it fail to accomplish that task, but it actively works
better to support something that is supposed to be false.
Like...if we turn this into a real-world thing, it would be as if a physics professor assigned a lab to her students that was supposed to demonstrate the conservation of energy, and instead ended up giving the
appearance (under rigorous and well-performed scientific analysis) that energy can be created from nothing, repeatably, every time they conduct this experiment as presented. That would be an absolute, unequivocal, abject
failure of an experiment! It has not only failed to demonstrate a true thing, it has convinced the students that the exact
opposite of it is true!
Edit:
Incidentally, this is one of the reasons why I try to avoid over-prepping as much as possible. If I haven't got prepped evidence to examine, then it literally cannot be the case that I am giving them evidence that doesn't "work." The players and I are discovering, together, what evidence "works." When I
do prep evidence in advance, as I did for this murder mystery, I put a
great deal of effort into ensuring that, though it may be a challenge, it should be possible to piece together the truth, just like a good mystery novel. Otherwise, one runs into the problem that Asimov was presented on the subject of science fiction mysteries: that is, that it is not possible to write a "fair" sci-fi mystery, because the author can always pull out some bogus technobabble to justify whatever conclusion they want, and the reader can't really argue because that's how sci-fi works. I don't want my game to
ever be the tabletop equivalent of a bogus mystery novel, so I either avoid unnecessary prep (so that I'm learning the truth right alongside my players), or very thoroughly review my prep to make sure it "works."