Merlow and the Shadow Queen worked together.In chapter 24, Merlow and the Queen duel. Merlow says that if he wins, he considers everything between the two over. Now, it oculd be romantic, but given that the Shadow Queen is Obviously Evil, it could be that they worked together. If they did, then that explains how Merlow knows quite a bit about the Queen.
- Jossed, given The Reveal about his past in Chapter 62. When Merlow had said he would "consider everything between [them] over", he likely had revenge for his father's death in mind.