The 4-koma sets at the end of certain chapters were what made this bearable.
I assumed from the beginning that the real girl was the one in the chair, due to their physical similarities and the comment about the other girl's voice being off in an early chapter.
Other than that - hell, I liked having a villain that admitted they were being a shitbag, admitted they were being totally irrational, and still pulled their shit while recognizing that they were even more guilty than the people they were killing. That was interesting and enjoyable.
The "oh, she was my cousin" twist sucked, although it explained why the killer treated that girl worse than the others, and the inciting incident - ok, shit, I've been in accidents. I've helped people in accidents. Unless the situation is really crazy, your first priority should always be your own safety. Yes, these kids could have saved two innocent lives by staying on the bus. That could just as easily led to everybody on the bus dying if their weight didn't counterbalance the father's. Some of them were dicks about it, but that was not necessarily the wrong decision to make.