Re: question
I appreciate everything about this answer. And yes, I absolutely believe she wouldn't put up with his issues either - right back when the event ended she was half-ready to smack him up the head for always dismissing his injuries before she realised, oops, not my place to do so anymore.
In canon he tends to only do so to with people he's about to kill (unless you count his silly pun of a catchphrase which, in Japanese, is him blatantly admitting he's a demon), but considering people in Kiseki are generally very used to, and accepting of, the supernatural, I think he a) won't be able to hide it for much longer (he's really not subtle about it half the time, and the only reason people in canon don't figure it out is because they don't realise it's possible for him to be anything but human), and b) will also stop seeing much of a need to.
His biggest concerns about people knowing what he is would be that it'd give them an unfavourable impression of either Ciel or of Sebastian's work efforts. An example: he doesn't care at all that Stiles side-eyes him, but if he suddenly decided he wouldn't sell Sebastian's cakes anymore because what if Sebastian poisoned them? That would be unacceptable. Stellar priorities.
As for how to get him to reveal he's a demon... generally, the more direct a question, the more direct an answer is the way to go with Sebastian. He can be surprisingly honest if just asked outright.
Have there been any major adjustments Stiles has had to do about his attitudes/knowledge about supernatural things since coming to Kiseki, and if so, what?