I don't agree with this.
(I mean, off course you can define "sincere mystery" by stipulation, and then your claim will be tautologically true; but given that you (i) are intending to capture something by the phrase that we all have a sense of independent of your coining of the term, and (ii) are intending the rest of your post to have explanatory power rather than being mere tautology, I am comfortable saying that I disagree.)
One reason I disagree is because your (2) is not coherent. You label it as a truth within the fiction; but then explicate that in terms of non-fictional states of affairs like
when the fiction was written.
To see what I am getting at, consider the character Wolverine. Obviously, it was always true within the fiction that he has some-or-other given name. But the readers didn't know that name until Chris Claremont wrote a little exchange with another (then relatively minor) character who addressed him as Logan (I believe this was in Uncanny X-Men 140, the Wendigo issue). The fact that Claremont didn't know what name he was going to give Wolverine until he wrote it, doesn't mean that there was no mystery about Wolverine's real name.
Perhaps by (2) you simply mean,
pre-authorship of the answer to the mystery, prior to the
play of the mystery starting. Which has nothing to do with in-fiction matters and all is about procedures at the table.
If that is what you mean by (2) then it is false: it is possible to have a mystery which is genuine but not pre-authored. Your analogy to a novel, where the answer can't be a mystery to the author, is inapt because it is possible to have a way of providing answers that is not analogous to writing a novel (either solely, or collectively as a group).
Obviously I am not
@AbdulAlhazred. But I think the answer to your question is much the same as the explanation, in response to
@EzekielRaiden, of how an answer to a mystery can be generated without it needing to be pre-authored and yet without it being simply authorial fiat in the moment of play.
To be clear, I don't have in mind here the Brindlewood Bay techniques, which I'm not that familiar with. (I've heard a bit about it, but have never played or read it.)
The three points may not be perfectly coherently described, but the fundamental disconnect remains.
There is a clear difference between any two of:
1.
Portraying Sherlock Holmes in a stage adaptation of
Hound of the Baskervilles (or any other specific mystery)
2.
Reading a Sherlock Holmes mystery you've never read before, trying to figure out the real perpetrator as you read
3.
Writing a Sherlock Holmes "whodunnit" yourself, which you hope someone else might enjoy reading and solving
Only #2 actually involves the personal act of "solve a mystery". #1 doesn't involve any solving because the task the person is attempting to complete is
performance, not
mystery-solving. #3 doesn't involve any solving, because the person in question already knows the answer in advance; they are instead
producing a mystery for someone else to solve.
I don't see why these three don't correspond to the pure "no-myth" detective game/experience (you're
portraying a detective, but you aren't
doing mystery-solving; you're telling a detective story, but you aren't
doing detective work yourself), a "some-myth" detective game/experience (you are both
portraying a detective trying to solve a mystery, and also
doing mystery-solving yourself), and finally a player-authorship of a mystery and then deciding what the solution was on the basis of past fiction and the like.
I'm thinking of the techniques that are used in a game like Burning Wheel or Apocalypse World, which can oblige the GM to narrate things under constraints, which include constraints about following from the fiction already-established in play.
I have never seen any of these things that actually achieve what you describe--doesn't mean they aren't there, I'm just not sure what in particular you're talking about that could achieve this.
Separately, I don't see how those constraints lead to the thing that seems necessary to me here. To solve a mystery as a player, the player needs to be able to reason about evidence, verify whether that evidence is reliable, and build a case that pursues the truth. How can there be a truth if it's genuinely not determinate until someone declares it so? Whether they declare it in a way that conforms to prior experience or not, it's still being
declared, not being
revealed. Whether there are or aren't rules that shape the choice, if there is no truth, there can be no mystery.
Like...consider a math test. It isn't and can't be a test to the person who
wrote it, because they need to know what the answers to the questions are in order to grade the test. It
is a test to a student, because there is a knowable answer (against which their performance can be compared). You could, of course, have a game where you are acting as a student who is taking a test, and follow rules and the established fiction to decide what the warranted result of finishing the test would be, but the player isn't
personally doing the process of take-a-test, they are narrating a story
about someone who is taking a test, and what consequences might follow from the taking of that test.
I'm not saying this is in any way an invalid way to play games, it absolutely is perfectly fine. But I can entirely understand why a person would say, "I want to be
personally doing the task of solve-a-mystery, in addition to playing a character who is solving a mystery."