I've been wondering if this might be the case for a while. In her manic state, Nodoka says that she's infected with thoughts from a "second me".
That "second me" might be Nodoka's upside down reflection, given her interest in the topic.
Shiori is Nodoka's "twin" and is responsible for her actions in her manic state. I think that's because Shiori is the "second me" who is infecting her thoughts.
The thoughts that she's apparently infected with are causing her to write things connected to the upside down house on her board.
And to start talking from the perspective of an upside down house resident while she's digging at the abandoned school.
So if these thoughts are coming from Shiori, it makes sense that she's in the upside down house.
I'm still unclear on who exactly Shiori is, though. If she's Nodoka's "twin", is she a daughter that Kyoko had in the house? If she's just "another Nodoka" why isn't her name also Nodoka? The versions of Kyoko in Kumon-mi and in the house have the same name (or maybe they actually are the same person and the scenes in the house are from the past?).
If she's the same Shiori who is Himawari's coworker, that means that she's not stuck in the house like the main residents seem to be. Does Himawari also have a connection to the house? Is she the girl with blue eyes from Nodoka's dreams and the waterlogged journal?