Personally, I think a succubus's power to change her appearance would encompass glamor for clothing. Clothing is something a lowly
doppelganger has to worry about, not something as powerful as a Demon of Temptation and Confusion. I mean, if her target's desire is a schoolgirl, she shouldnt' need to run out and buy the uniform, she should just bamf into it.
One thing to keep in mind: she is a demon. She is not alive. Therefore, there are likely things that the living would leave around that just aren't
there. The bathroom hasn't been touched in months. There are no hairs, fibers, etc on the floor, no grooming kits, no nothing. A living shapeshifter would at least leave
something behind, because it has hair and skin and bodily functions. Pointing this out might suggest to the PCs that the noble is an illusion, or something else (vampire, lich, etc).
Books on where and what she's impersonating (research material for things she should just know).
I say this should go in there. Extensive notes on the noble's history, his likes and dislikes. In list format, or in bite sizes for memorizing. "Why does the noble have listed how he likes to order his roast duck?"
A year, eh? Plenty of time for the succubus to have conceived a child by the now-murdered Count. Perhaps she wants the little half-fiend (alu-demon) to lay claim to the Count’s inheritance.
Or the
corpse of the alu-demon infant. She's just as likely to just kill it than leave it alive.
Some kind of forbidden/banned book -- an erotic novel (or series of them) written to entice readers into depravity. The books also contain encoded instructions for rituals to entreat and honor Grazzt, Socothbenoth, Malcanthet, or whichever demon lord she serves.
Or books that either 1) have bound demons/devils in them, intentionally made to possess the reader, or 2) powerful Suggestive magic that isn't just entice, but literally corrupt the reader. Hell, she could be
writing such a thing.