So the thing about this concept is that SI!Muzan pretty much abandons the plot early on. You know, on account of Tanjiro never becoming a Demon Slayer in this AU.
(They make extra sure of it, feeding Nakime a bit more of their blood so that she can use her floating eyeballs to keep an eye on the cabin and its inhabitants. If a Slayer or a Demon were to come near and try to talk to them, she is to notify SI!Muzan immediately.
A Demon, they kill. A Slayer... well. That depends.)
Because of his absence, a lot of things that go right in the story start going very wrong. As in, main characters get themselves in seriously unfair fights and are in serious danger of dropping like flies.
Obviously SI!Muzan doesn't want this for multiple reasons. Moral reasons, plot reasons -- and because they need a fuckton of Slayers for the final battle to convincingly fake their death. And make the final boss fight look cool as fuck.
(They always thought the Canon!Muzan fight was lackluster and not as interesting as all the other ones. Just a bunch of panicked flailing and tentacles waving around. And ugh, his final form? A crying baby?!
Nah fuck that. Their boss fight is gonna be legendary.)
So they find themselves... stepping in so to speak. Not literally, but in minor ways that the average Slayer wouldn't be able to detect. Speaking in the minds of Demons to distract them, having Nakime summon Slayer's unconscious bodies away from danger, that sorta thing.
Eventually people start to wonder what's up. Obviously a Demon helping them is out of the question (they don't know about Tamayo yet), so they start wondering if some mysterious Slayer is involving themselves in the Corps.
I'm thinking that it initially leads to some secret identity shenanigans -- up until SI!Muzan gets found out to be a Demon.
(They don't find out SI!Muzan is the Demon King -- they just think they're some rando Demon, lower than the Twelve Kizuki since they don't have the special eyes.
Oh the irony.)
And at that point the Corps becomes under the impression that the Demon King is sending secret and incredibly human-like spies to ferret out their secrets and Breathing Techniques, which sparks a wave of paranoia and witch hunts among its numbers. Kagaya does his best to calm everyone down and suppress this info from getting out -- but it still does some serious damage to the Corps morale.
A running theme in this fic idea is that SI!Muzan's attempts at being good only end up making them out to be some sort of demonic mastermind. Like, they end up being massively more effective and scarier than Canon!Muzan purely by accident.
(I am somewhat (okay a lot) inspired by SIkuna's guide to not fucking up perfectly good children created by the lovely @overly-verbose when it comes to these sorts of perception shenanigans.)
A lot of it comes through on their behavior. SI!Muzan comes across as a lot more calm, cool, and collected than the Canon version, never making unnecessary moves and acting completely unbothered even when things don't go their way. They never raise their voice, never lash out at others, and never get violent unless provoked. If anything they're friendly and charismatic -- which is really fucking eerie to everyone.
The Demon Slayers because they're not used to such calm and seemingly harmless Demons, and to the Demons because holy fucking shit, why is he acting this way, is it a trap, is it a test, is he gonna kill us, what the hell do we do?!
They really give off the sense that they're up to something -- and that they know a lot more than they should. And it really, really, really isn't helping that SI!Muzan invertedly drops lore bombs during random conversations -- lore bombs that seem like threats to the Demon Slayers nearby.