I took the idea of assuming each time we see any green haired girl acting with true kindness towards Keiichi, or acting embarrassed/flustered/guilty that's *not* the heir and that each time we see a green haired girl acting cruel, mean, or negative somehow that is the true Sonozaki heir.
After rewatching the Watanagashi arc assuming that the Mion in the beginning is NOT the heir and that we're introduced to Shion for the first time in the motorcyle gang scene outside Angel Mort and that she *is* real heir most of the storyline in this arc makes a lot more sense (though it's still way convoluted at points
). Each time one of them repeats a question to K1 they're really trying to figure out what the other sibling was up to and how much they know or testing Keiichi.
For example: When everyone meets while setting up for the festival the Mion there tries to take Keiichi away from Tomitake/Takano/Shion but Shion brings up the murders and Keiichi gets drawn into the explanation. At this point someone had informed Shion about Mion liking Keiichi (Rena or who???) but the Mion here says she hates Shion even more now. If, at the first Angel Mort scene, the waitress says she and her sister talk all the time about Keiichi those 2 statements conflict so I believe this was Mion/NOT heir dressed up as the waitress trying to cover up how she knows who Keiichi is. Going into the storeroom in ep5 that has to be Mion(NOT heir, true Shion, whatever) as she doesn't seem to know the gory details of Watanagashi and she seems nervous once inside, standing behind Keiichi while Takano explains the history of the festival and other reasons below. Takano says that Shion is understanding and is the reason she confied in her...Which "Shion" is it, the real heir, or not heir in Shion's clothes? I think it's the real heir and not the person in the storeroom with Keiichi but that doesn't quite fit with...
The Mion that shows up with the group later after Rika's performance is the true heir in disguise as Mion. She somehow already knows that Keiichi was in the storehouse and my only guess at this point is that Takano or Tomitake somehow told her and that they were all in on some kind of pact to get either Mion/NOT heir and/or Keiichi to wander into the storehouse to pull a demoned away act on them. It seems reasonable that the true heir is jealous of her younger sister. If the lock was recently changed and Rika's in charge of that she would have to be involved somehow as well.
The first phone call in ep 6 where Shion is on a bed is certainly a different room than the apartment where "Shion" dies at the end of ep8. It makes sense that the person on the phone was Mion/not heir in Shion's clothes.
The second phone call is the heir...though showing a glimpse of a bloody Rika-chan is pretty interesting. The 3rd call at the end of ep7 where Keiichi calls her out on confessing to the village chief is the heir/crazy one we see throughout ep8.
In ep8 when Rena and Keiichi go to the Sonozaki household it makes sense that Rena knows where to go since she's been to Mion's house before but it's interesting that the torture chamber is on the main grounds while "Shion" supposedly lives in that apartment elsewhere. You'd think with all the torturing she does she'd live closer or that Rena would have seen her before.
When Rena asks about how Mi-chan saved one by her own will the heir responds by saying "Who knows? I wonder why I didn't kill
them." Is this referring to Tomitake/Takano, a translation error or? Why does Rena shush Keiichi while the heir/Mion is confessing to the murders?
While taking Keiichi down into the torture chambers she talks to Keiichi about how he saw (*not* we saw) the various torture devices in the festival storehouse. This furthers the idea that Mion/not heir was dressed as Shion taking Keiichi away from Rika's performance in ep5 and Mion/heir covered up Keiichi's slip when telling Rika-chan he saw her whole performance.
I definitely agree with everyone on the true heir switching places with Mion locked in the cell and tossing the not heir's body into the well after she shocked Keiichi. However, what I'm not getting is that Oishi never mentions whether one had a dragon tattoo on her back. If Mion was found in the well by the police and Shion was tossed or jumped out of her apartment, it's either the same corpse and the true heir is out there or Oishi isn't telling Keiichi everything (maybe he thinks it's irrelevant?).