I don't see why waking up from unconsciousness in some sinister person's house is any different, in its fundamentals, from these. (EDIT: And my memory was also correct, that I said nothing about who put the PC in the manor, nor who might be guilty of anything. All I did was describe waking up from unconsciousness in a particular, recognisable and salient place. Which as I said seems to me like standard AW GMing.)
I don't really get how one would wake up from unconsciousness in such a place and
not have it explicitly mean "this person has been up to no good." Like...sure, maybe they aren't guilty of one particular thing you thought they were, but
especially as the result of a failed roll, this screams "suspicious manor-owner IS guilty!" to me. And that looks exactly like a blatant use of force: "I'm
making the guilty person be this one and no other."
Suppose that the player start declaring actions for their PCs with the goal of finding out who is supplying the cult with weapons. I don't see how that is - in structural terms - any different from them deciding to start declaring actions with the goal of finding out who killed such-and-such a NPC.
No one is supplying them; they've been
stealing them. That's why the PCs were there in the first place--a player decided he needed a magic shield, so they went looking for one, but a 6- on Supply revealed that raiding on weapon caravans had heavily affected all weapons trade (but especially magic ones). They sought out a prominent weapons-merchant (via prior contacts), who offered money and a fancy magic shield as payment for solving her "my caravans are being raided" problem. The party took up the offer and collaborated with her to make a trap caravan with only themselves in it, so they could catch the thieves in the act. I used this golden opportunity to explore the cultists more, as the players had pretty thoroughly crushed the evil druids and this stuff fit the fiction about the cult very well but made less sense for the other fronts.
They have sometimes used the records they recovered from that place to investigate other locations of interest related to the cult, but their priorities have often been elsewhere and will probably remain so for a while. (I prefer treating such delay not as "aha! you have failed and now I will punish you!" and more as "this explains why threats tend to escalate; the party faces tougher foes because they tend to have had more time to prepare.")
In thinking about it, perhaps I've come across what the issue might be. (I want to emphasize
perhaps; I still am not sure this is more than a gut feeling that has no rational explanation.) They feel like
different types of ignorance. There is "ignorance-what/who" and "ignorance-why." "Ignorance-what/who" reflects a lack of understanding about the chain of
events that produced a certain result and/or the
persons responsible for those events. But the chain of events had to be whatever it always was, and the persons had to be whoever they were. It feels wrong, to me, to say explicitly that it is not only unknown but
unknowable what the chain of events was and who conducted them up until the point that it is declared, and then forever after that is exactly what it always was and never anything else. "Ignorance-why" reflects a lack of understanding of the
motive or
purpose behind some event or behavior; in principle, you already know
what the event was (e.g. in your example, someone getting possessed) but are trying to figure out the motive behind it. It feels perfectly natural, to me, that you not only don't know but
couldn't know what a person's true motive was until the moment of a shocking reveal, and I have no problem with it being a declaration.
I would argue that we can see some examples of this discontinuity playing out in ordinary fiction. Leia was established as Luke's twin sister in
Return of the Jedi. But as early as the previous film, the writing includes stuff like her kissing him, which becomes deeply awkward if they were always intended to be related. It's pretty clear that they were inventing the story as they went along, retconning what they needed to (e.g. consider Luke's deep frustration at Obi-Wan's "from a certain point of view" line) in order to keep things going.
That is the kind of problem I have with declaring the real killer or the like. But plenty of stories, whether singular or in series, have ambiguous character motives (be they heroes, villains, or something else) that only get revealed later. Such very rarely, if ever, require any actual re-alignment of any facts-of-the-matter--the events still played out however they played out. What has changed is the audience's
understanding of the events, not the events themselves.
So while, yes, your "we found black arrows" is
theoretically an alteration of the facts of the matter, in practice the only thing it actually changes about those past events is what the players
know about the
context of those events. The brother wasn't pure as the driven snow (unless "driven" means "driven-upon"!), he was already morally dubious. That doesn't make any part of those events play out any differently than they did before. But it does mean the characters must now reconcile their original beliefs with this new information. It was still the brother, still defending the tower, still against those specific orcs, still being possessed by that specific demon. The causal chain remains unmodified. Only the context in which that chain occurred has changed. That feels acceptable to me. But changing
who the demon was due to a player (or DM) declaration triggered by a successful (or failed) roll? That's now actually altering the events themselves, not just the context surrounding them, and that would bother me.