It's probably reasonable to assume that Otsuyu will show up at some point as the third of the great ghosts, but at this point I'm not convinced the ghost in the windows is her. None of her descriptions match someone with a disfigured face or even any wounds at all. If anyone fits, it would be Oiwa as she's depicted in many paintings and plays. The woman from the shrine who we're introduced to as Oiwa stands out too much from other spirits, too. She's the only one so far that actually has legs and no problem at all interacting with physical objects. And what about Oiwa's ghost story would give her the power of creation?
What I'm speculating Yuino is going for here is that both the fujo and window ghost are different versions of Oiwa, one the spirit worshiped at a shrine who's gained some aspects of the god Inari and the other from the ghost story, since apparently the real-life events that inspired Oiwa's ghost story happened to someone else. It could also explain why only otaku Oiwa can hear her.
And while I'm at it, the scratched-out name is of course also Ryouyo but spelled differently as 了誉 instead of the protagonist's name 涼世. The Banchou Sarayashiki story mentions a priest 了誉 who was called in to exorcise her.