I like what they were going for but a little too confusing. That said, I have a theory I like most.

The theory I found most convincing is: They're all dead. And this is purgatory. They all died when light went out which you see in the 13th episode. A meteor or whatever - not the first time Japanese stories take such approach. Also note that everything in their order turned worse after this event. They are stuck in purgatory with their sins. The purgatory is built based on their sins so they could face them. That's why we see their sinful actions also being realized easily, they are in the world of their own making (and people around them) made explicitly as the purgatory for them.So why Miho is missing - she wasn't a goody two shoes either - well she commits suicide inside the purgatory world already. How is suicide repentance? That wasn't shown clearly but what was shown was that she ended up in the wheelchair after turning worse, lost her power and then she put up with the bullying, ignoring it. Somewhere along her way in purgatory, she had already paid for her sins through suffering.And this helps explain why girls were all locked in the room and tortured, if they don't repent, then they will suffer until death. Keep in mind this purgatory is constructed with sins of the people that are dead at the time so it all works out in connection, as if led by fate. The director of hospital and Okazaki covered up accident which lead to Miho being pushed over the edge. Even Kage ends up suffering. If everyone died during blackout then how come Miho's closest friend wasn't there but instead shown as dead. Miho admired her friend's sense of justice so perhaps she really had no major sins to repent within purgatory - Miho is just someone who tries to copy the purity she saw in her friend but was never as pure as her.Recall blue dark room interviews. Why were they there, why did they speak openly to us directly. It was omnipotent presence they were talking to unaware of it. HOWEVER, it's a little questionable how come Kage doesn't face more challenges. Notice the water started filling in the ship in later vanishings - this is because the purgatory still follows most laws of physics. Ship hit something. Filled with water - they heard screams and scratching - the people that vanished drowned - yes they had repented or suffered enough at that point but it was by purgatory strings of fate. Effectively making Kage responsible. But we can only assume, perhaps Kage's repentance was very deep. Why I think there is more to Kage's repentance is because she disappeared with light glitch which likely concluded her purgatory - perhaps her life was suffering enough and vengeance was the only sin that needed to be paid for.Also, rewind plays a role too probably - it looks like girls had the opportunity to live through their lives again or look over it before moving on. I lean on that they were reviewing their life as they were talking to us at the same time. When they moved on to their common heaven, it was graduation before whatever else happens there.Interestingly, right before Mei was suspect of being a mastermind, when camera was looking at her, it had an orange light on side of her. Was this to confuse the viewers? Or did it mean something? Mei perhaps didn't have many sins to pay for except turning a blind eye so she is there too - she also expressed pride and jumped to accusations. She gets to deal with both - she is being accused of being mastermind without being heard out and she then breaks down crying how she is actually insecure. Perhaps that's all she needed to move on.But why orange light on Mei? I don't think just because she was "purer". Why...why...why... this will sound far fetched but her being most grounded, actively seeking solutions, observing, trying to ease others, avoiding to fall to behaviors led by emotions and effectively leading others to remember and repent...I believe the light represented her being supported/led by hand of purgatory fate as she was instrumental helping others. She didn't need to be there any longer but purgatory needed her.

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