I like your theory
Well, I don't know exactly, but about the time-leap, besides there being more than one leap like you say, I also keep in mind the possibility that maybe Hinata is not the only one that comes from a future.
About the Senpai with glasses (Shinohara), there's a detail: in the funeral, he seemed not to know about Sora being found behind the wall (Hase and Tokiwa comment that he and Hinata seem to doesn't know about it). However, when he alerts Hinata about the painting containing some hint to Sora's murderer, we see he did know. Maybe he just didn't tell Hinata, or maybe there's something to it.
Ok this is not spoiler, just my theory for why Sora is acting so weird!
I think that maybe Hinata originally died instead of Sora, and then Sora went back in time and "fixed things" but ended dying himself. That created a new timeline where the story is set, and now Hinata is trying to fix things!
No clue who killed Sora tho, I feel like none of the artclub people did it, maybe the person involved hasnt been shown yet? I am curious too about the glasses senpai, like he was the first to seem innocent since he died in the funeral, and usually in detective shows the first person to be cleared tends to be the criminal haha