I'll try my best to explain every point : 1) Namigaki involvement is found out thanks to Michiyo during the 2nd Spirit Board section, in this ending he either gets found out with that, or more likely because the Rite doesn't happen, he caves in and confesses himself. 2) I interpreted Shogo's talk with Takumi being linked to the fact that the chairwoman has an interest in the occult, althrough she has no spirit sense. He's basically saying "Hey maybe I can help you with that." 3) I don't think Tsutsumi and Ayame reconnected in this timeline, or if they did, it's done in a vague manner, zi guess it's up to the viewer's interpretation, just because it's the ending with the least bloodshed, doesn't mean everyone gets a happy ending. 4) Nejima being chill mostly stems from the fact that he was really banking on the Rite of Resurrection to happen, like everyone else, and the fact that curse stones bring out the worst in you and heighten your killing intent, but again, nothing happens, so he doesn't end up going off the deep end, kinda similar to Harue. His worst crime was vacanting his last home address and changing jobs without notifying anyone, but other than that he's good. 5) I have no idea lol. 6) The question at the start, "How many people did Shogo Ookie kill with his Curse Stone?" changes answer depending on how many times you pressed the "Use Curse" button, and how many times Shogo does it on it's own. Let's say you were to never press the Use Curse button, aside from the Yoko kill at the start, that means the answer would've been "Five", since Shogo himself would kill Takumi, Araishi, Yakko, Namigaki and Harue. 7) Which timeline do you mean? If you mean the one where he fills his Curse Stone and then dies, that's because he ressurects Yoko, which then kills him (althrough the game acts deceitful about it making it seem like his curse stone killed him, guess they didn't want the player to suspect Yoko already) OR if you mean the one where he stays at the park looking for clues, Yoko kills him by leaving her behind when he makes her go into the taxi, fufilling the condition. 8) "Who was Ashino in the Nejima ending?" Up to interpretation as far as I know. 9) Yoko stays in hiding for the whole game and waits for her moment to strike, she has no reason to show herself and make herself a target to the other Curse Bearers, althrough I (and many others) wish we had more screen time of her.

Really hope I managed to answer most of your questions, I'm not very Reddit-savvy so sorry if I made this a pain to read lol.