I agree with your interpretation of the old one. It makes sense. Especially with the corner dialogue you mentioned,
and how in a way the old one appears in the corner(?) last update arguably i'd say.
I have heard from word of mouth and others experiences that kids having imaginary friends either be it from trauma, to cope, or it being a healthy thing for them to have practicing social skills. I have not read any studies about it though.
It feels weird to think the old one is in tandem with pareidolia especially with it being a crow mauler last update, but would answer(?) to who was narrating the final event this update. It feels way too formal to be Himawari talking and no where near cocky/wicked enough as when Sekai is talking.
Though this does leave me still guessing to whoever was "???" during Not the Nightingale event and with how each the returning to form line changed meaning when the CGs would change each time. The middle Ami dialogue could be a bug I had to click twice to read through the event.