終わり！
Summary, spoilers ofc
Once again, read mostly on trains and while moving around, so I haven’t been able to focus in as deeply on specifics or take notes like I probably would otherwise do. But the writing style I’m still very much liking, it feels very very flowy. Usually I might stop after 20 pages straight because a book is starting to feel a bit same-ish, but this one if I stop after that much, it feels like I could just immediately keep on going and going. I liked the kind of quite skittish deviations in conversation and bit of humour when 遊馬 and 月夜 were talking, and also the references to more modern mystery-stuff like Knives Out or Sherlock tucked in there with the 本格ミステリー and all of the Japanese stuff. It’s really coming through how big a mystery-buff the author is, feels quite meta-meta.
No more thoughts, will just piggy-back off of other people putting in more effort than me into trying to understand 
Personally I don’t see her as a candidate at all, she seems too set up as the kind of key behind investigating it all. It would really surprise me if she was responsible, but to be honest essentially any other character wouldn’t surprise me. Although there is the element of her having gone down (presumably) to see 老田 before going up to talk to 遊馬. But the fact she was so open about that removes her even more in my eyes.
Good catch with the bright light, I’d forgotten about that. The fact that it was explicitly mentioned makes me think it might be more of a possibility.
I was thinking that there’s got to be some link to 神津島 and the unreleased mystery. Random throwing it out there, like it was something he’d set up to go off after his death, whenever it was. He seems like the kind who would make sure his servants went down with him if it meant a nice mystery to solve (speaking of which, was there some mention of Ancient Egypt and that happening with pharaohs earlier on? I’ve definitely read something about that in a Japanese book this last week or two, but my brain is so fuzzled I don’t remember if that was from this or not. If it was from this, I wouldn’t be surprised if that was the case).