The Ouka, the real one that killed herself, seems to have been an actual idol. -and through implications something happened during a concert. Possibly something that resulted in the death of Chihaya.
And thus she locked herself in her room where she tried to hide away from the world and prying eyes.
Possibly she loved the real Chihaya and couldn't live with herself, knowing she'd never be able to be with her. A lot of pages in manga seems to point out that the Ouka is an idol, from the guy who literally becomes the reason for her loop. To the start of the loop being the train, where the last moment is her imagination killing her. I'm actually inclined to say she is FULLY dead, and that the cat and mouse play is her post-death imagination. A loop created by her love for Chihaya and her hate for corporation. Ouka literally made a whole part of her brain dedicated to a school where girls fall in love. She can't escape the loop, because escaping would mean moving on, which she can't due to killing herself, because she will always know, at the end of the day, she killed herself for Chihaya.
If we take all this into account, the insane girl who shows up at times, are Ouka, accepting the insanity that surrounds her world. Possibly the "tranny" is also one of Ouka's later added idea, due to her not recognising the Ouka in the picture. Since the Ouka in the picture NEVER was the real Ouka.
You could actually say that the ending is not her imagination killing her, but herself, like you said, but not because she wanted to willingly go into a coma again, but to escape the reality without Chihaya, Ouka went insane at the end. She hated the job that caused her the deep rooted trauma so much she ran after Chihaya, but she can't get Chihaya. Because Chihaya is nothing more than a post-death illusion of what she imagines her to be. Which is probably why she made her own jail cell where she spent a lot of time seeing different Chihaya come and go. The Ouka at that stage is the one who still blames herself or possibly hates herself for the world she created.