I don't think author even wanted to explain time traveling, dunno which eyes are u referring to but i suppose the ones from the first timeline where he killed his clubmates, i think its cuz he has gone crazy, and the clue was about the painting being a homage to the artist who killed someone
You missed everything, everything has been clued, the reason hinata regressed is because sora pleaded for help and it reached hinata (try to watch boku dake inai machi, its kinda similar plot with the mc regressed to the past and there was a reason he got sent to the past)
As for the painting, its the death motives filled by the corrupted hakuraku
Is this the conclusion? This is from absolutely fucking nowhere, nothing got explained, why the protagonist time travel, the horrid shack painted with blood, why Sora has those strange eyes, the clue on the painting, nothing or I’m just missing something.