Of course Eye Scream is a staple of the murders—Rumi didn’t want anyone to see what she’s really like. For that matter, Me-Mania's probably well aware that he's not a very pretty sight either.
The red dress Rumi has as the Real Mima wore during her confrontation with Mima. She said it was new, but maybe it was what Rumi wore back when she was a pop-idol.
The dresses Mima wears are associated with the Color Motif of Red.
Mima's outfit as part of CHAM is pink. Pink = white + red. This means that she's pure, innocent and the insanity hasn't taken hold on her yet. Mima is also hallucinating being mocked by herself in said dress.
The outfit she wears during the rape scene is white but with red accessories, showing when the insanity starts to take hold.
Last, Rumi is shown off as "Real Mima" in a red dress that symbolizes the Sanity Slippage that Rumi has gone into.
The color red is commonly associated with sexual maturity and forthrightness when worn by women, possibly because of its association with romance, or because it's the color of menstrual blood. In some cultures, red is also the traditional color of a woman's wedding dress, or in other words, an outfit that marks a transition to adulthood and married life. As an idol, Mima is made to dress and behave like a little girl despite being a young adult, but after becoming a professional actress, she's suddenly being pressed into doing "mature" material like filming nude photoshoots and rape scenes.
After the reveal, everything that happened before now makes sense. Rumi had access to Mima's room, she could spike her drink, install cameras, kill her fish, and many more.
Early on in the film, Rumi brushes off the exploding letter as a prank. A letter that left Mima's manager bloody and curled up on the floor dismissed as a mere "prank".Of course she wouldn't want Mima getting too suspicious, either wanting Mima to keep her guard down for the next attack or being pleased Tadokoro got injured since it was his idea for Mima to start acting (or both).
One of the worst parts of the rape scene was seeing Rumi crying while watching it being filmed. The Fridge Horror comes in when you're given time, near the climax of the movie, to remember that scene: you realize that Rumi may already have been unstable and delusional but this and the racy photo shoot were the things that pushed her permanently over the edge.
Or even worse, if Rumi was already delusional, she thought she was seeing herself getting raped. It gets worse when you consider just how strongly she protested Mima doing it in the first place and how it connects with the conclusion of the Show Within a Show.
Some of the the lyrics from CHAM's song at the beginning of the film could have given Me-Mania... ideas.
In the climax, most of the chase scene is a delusion of idol-Mima chasing the real one, gliding and hopping like a fairy. Except the Glamour Failure reveals it's Rumi chasing down Mima, visibly panting and running in real life. First, props to her for having the stamina to keep up with Mima who still has her dancer's training and second, she was able to keep up with Mima long enough for the gliding delusion to happen. The glamour is also unsettling with idol-Mima having a Slasher Smile that we've never seen on the real person.
