Thanks for clearing up some things. Also I loved the character development information, always a treat to see how a design is developed!
If Jester is Betrayals and Nightmares (and designed like a court jester, where nobility are often/depicted as backstabbing intrigue) and the 'older brother' looks like a noble himself then maybe 'Eliam' is the Demon of Manipulation and Paranoia?
Ending 3 information paints a very fun image of what things might be going on with everywhere in the setting: it'd be a cycle of belief rising and waning and demons taking advantage of the lower times. Naturally I imagine there'd also be a lot of local folktales and superstitions that develop every time the metaphysical barriers start to fall.
Ending 1&2, while I am sure that the Qualian nobility are just bog-standard human jerks (you do not need mystical manipulation to do horrible things to people), I can also imagine there's a demon among them happily using their court as a buffet table. ....probably was Jester actually, but I can somewhat see him sharing his feasting grounds with his 'brother'.
Sequel: I'm excited at the idea and hope development goes as smoothly as possible but I have to say the synopsis actually made me laugh. Starts off very "dark fantasy vn" but quickly drops into comedy/parody. I can picture it too: three romance options and a fake (trying that route is a definite death sentence) with a lot of bad ending paths, and the demon lurking at your back is the not-so-secret route whose responses are shaped by how you handle/react to the other characters. You don't know which romance option was involved in your death but it's a Morton's Fork because all the mortal romance options were involved in some way. One is actually feeling guilty about you "nearly" dying and wants to make up for it, another is terrified that you're pretending to not know and plan to make them pay, and the third is surprised and waiting for the perfect chance to finish the job (although depending on your dialogue options, they might change their mind and decide you are the perfect match to them in temperament). All the while the demon leans on the fourth wall to make fun of the characters and your responses to things, while sometimes deciding to "spice things up" for fun like killing someone and hiding the knife in your room.