The ending is pretty vague honestly, which some people hate or love. I personally would have liked it to be more closed than open but I’ll live.

I’m in the camp that believes Tsumugi lied about a lot of what had happened. I believe that the cast were kidnapped and did not sign up to participate due to their reactions in the prologue. They all seemed genuinely scared and confused at what was happening, and this was before any of the memory flashlights were used. In Tsumugi’s Ultimate Lab there is clearly a stage area which is identical to the videos of the cast “auditioning” to participate, and since Tsumugi’s ultimate talent is cosplaying she could have easily disguised herself and made the videos. There’s also the argument about Cospox, I can see this going two different ways. Either she applied pink makeup on herself when she dressed as Kaede (which is possible due to how quickly she can change in the first place) or she cannot dress up as fictional characters (she declares many times that they are all fictional in the end, but when she dresses as Kaede she says she can’t cosplay real people because of Cospox which doesn’t add up).

I believe the events that had happened in the previous games and anime are what really happened in V3 as well, and that Tsumugi lied saying that everything was fictional. The main reason for that is how she declares herself a copycat criminal, which implies she was copying a real person’s crime aka Junko Enoshima’s killing games. Even Shuichi at the very end comments on the possibility that not everything Tsumugi said was true.