1. In ch 8, Suzu (Chitose's 'contractor') and Nelly talk about how hard it is to lure 'her' out, even when 'she' was there. The same pronouns were used, so it's a bit confusing, but the next line confirms it. Nelly says, "She's seriously good at playing hide-and-seek."
2. Yuzuha's lingering attachment is most definitely Mashiro. Nelly wanting an inexperienced 14-year-old to join the society, even if there was a memory wipe alternative, makes complete sense now. She hinted at knowing of Yazuha's death saying back in ch 2 "You wouldn't want the people around you to wind up in danger because of you, right?" It's a line that is extremely manipulative, which struck me as weird before this chapter bc Nelly was pretty chill.
3. Look at the title. Look at who Mashiro has been writing letters to.
I could continue, but all of this pretty much confirms it.