I understand most of the movie but one scene I can't completely filter into different buckets of reality is, the scene where they say Mimorin is going to have another concert with the two other idols remaining in the group, and all the otaku gather and seem to collectively see Mimorin there (it was written in her blog I think that she would be performing, so everything thought she'd be there). Was it the chubby female manager or performed in her stead? How would no one have noticed? Or was it just Mimorin's delusion?
This is more-so speculation on my part, so sorry if that this answer doesn't suite well enough.
It is not true. Mima did not actually perform and the manager was not performing as her. The image that we see is part of Mima's delusion as she originally doesn't know that her manager is pretending to be her, and truly believes that the person behind the blog is some sort of demented version of her.
Because Mima is unable to separate herself from the blog, she sort of enters a state of psychosis and through that altered reality, she sees herself performing.
The crowd most likely was cheering on the group because it is stated earlier in the film that the duet the remaining two members did had hit the top charts. So it is probable that the fans are celebrating the group regardless of Mima being there.