Making a contract with Yuki-onna should have been the solution to the issue, but things didn't go as smoothly as I had hoped.
Because of the nature of the situation, I had to run everything by both my father and Renji-san. Thankfully, convincing my father went rather smoothly.
After explaining everything that had happened with Yuki-onna, he simply nodded and said, "Understood." That was it. It felt too easy, but I suppose my father believed that I wasn't lying.
The real issue, however, was Renji-san. He was much more concerned about whether Yuki-onna would truly avoid causing harm to Aya-chan, and he spent hours questioning Yuki-onna to be sure.
It's odd to call that an "issue," though. Renji-san's reaction was actually quite normal.
He and Yuki-onna—who had taken over Aya-chan's body—talked for a good two or three hours. I was there for part of it, but Shirayuki-sensei called me away to help prepare the contract circle, so I didn't hear everything.