I expected the answer arc to cover the same time period as Watanagashi but once again this series managed to surprise me. For the first time I could not wait and watched the raw first but didn't post here before watching the sub to get a clearer picture of the events. Impressive episode - I voted 9, not 10 because of the stupid incredibly strong athletes that seem to live in Hinamizawa according to the anime scriptwriters.
Spoiler:
I think it supports the "Shion was the evil twin in Watanagashi" theory. We see how easy she can get enraged, we see that she is not too fond of Satoko to put it mildly and she can indeed pass for Mion even among her friends.
However, she seemed to be genuinely surprised when she heard about the Furude family deaths so I guess we cannot explain all the murders by simply saying that Shion did it. I don't like Satoko that much so I don't really hate Shion...for now.
I still don't see the great significance of the opening scene, doesn't it just confirm what almost everyone suspected - i.e. The Evil Twin pretending to be the victim in the end of Watanagashi?
So Shion and Mion get on well at that point but that does not seem to be the case a year later, unless they just pretended to dislike each other in front of K1. I guess Satoshi's disappearance has a lot to do with the deterioration of their relationship. Satoshi's voice seemed kind of unsuitable for this character, btw.