I've played the game to the point where Rumah is safe for now and Gurtak is subdued with magic, I'am on Dasans Route. I haven't met umah yet. Is that what's missing?
Thinking now, Arianna moves around the "Original" Rumah, so if the new area exists and if Maui and Umah live there or not
shouldn't be important.
By the time "Arianna in Rumah" was added to the game, most people would have fought Threg, and the whole conversation of "I saw Zorad in the Orc Village" when you have dinner with Tia and Katherine didn't exists.
I don't know if there are others factors besides the end of the House Arrest that trigger the encounter with Arianna.
Is necessary to capture Umah and decide in which form she ends up for Arianna to visit Rumah? I don't know.
You can try having Roderick tell the truth about the Cult to his family and rise Arianna Affection.
The 3º encounter with Arianna just "happens" when you exit Arenfield by the north and go to the Bridge where Marcus was murdered.