Glad im not the only one who had trouble understanding the story and I am also a victim of having to google certain words despite speaking english my entire life lol. I really do hope the side story clears some of the story up and reveals more of Miriam's past, I feel like the game only went through Adelle's, Celia's, and Akira's.
For the endings im pretty sure there are 3 main endings and 2 main bad endings, im not sure how many death screens there are and which ones u already got so I wont say anything abt them yet. There is a walkthrough file in the game’s folder that could help u with that u just have to open ur downloads > operation eventide (folder not the game) > folder named “game” > “walkthru” then in that file it would basically show u how to get all cg’s and endings, it is a little vague when it comes to the first main bad ending so before u enter the library make sure to unlock all miriam’s dialogues first.
Note: Some cg's and one ending requires u to finish the game to unlock them
Goodluck with getting all endings and cgs!!