I typed out an essay on who I thought the mastermind was if they were a time traveler, but I hit a crucial flaw that I think probably clears Yotsuha, Uichi, and even Yae:
Time travel doesn't work that way in this series. From what we've seen, the only known way to time travel is through...
I mean... She believes she killed him because she wasn't there. Everyone she blames for Uichi's death doesn't have a direct causal line to his suicide. Most of his classmates are his murderers because they just chose not to do anything. In the same way, she blames herself for not being there for...