although I certainly didn't expect three people to target her at the same time.
I'm in the same camp as you, and since I was looking for other clues last chapter, this one eluded me in chapter 2, page 12:
Vague enough that you think the Fairy King is simply talking about one individual, not three. Number two is a bit more involved because
Butler's daughter is John's fiancee, so that's how the "family" relationship is established.
In 6.2, pg 17 Zachary makes his declaration that he'll find the culprit, and in the following two parts lays it entirely out for all three, with this little bit of (unintentional) reveal about the plot:
Zachary is still indeed a mother fucker, but he fulfilled his promise.
I hope the next chapter will tell us how she could force Oliver to spill the bean
From chapters 3 & 4
that was the purpose of the portrait she had made. To be evidence against the killer.
and how nobody realized that the window's lock was broken (p. 13) though in chapter 3 p. 23, Matthew was said to have purloined a spare key.
5.2 Logan confirms that the door was locked before she went to bed, and that the last two people in her room were Oliver and then Anne. The four stationed guards report that "no one else entered," but
(and this is what I feel is a little nebulous, but is what allows the scenario to work) not explicitly reporting that they both indeed left.
And that particular page says that one key is in the possession of Willis Howell, the Grand Chamberlain, with the other being "in a drawer in one of the other rooms," and Ophelia only postulates that an accomplice inside could have opened it, not that any particular suspect had a key.