>>6
Concerning Rei, my very first impression was that she couldn't cope with so much happiness all of the sudden, she wasn't used to it, and that, maybe, she though that as her life couldn't possibly be better, then she could as well disappear now while she was experiencing what she had longed for her whole life.

There's one thing that reinforce the suicide thesis, it's the preparations she made the day before : telling to Kaoru to get back her book, asking Nanako on a date, etc. As if she knew and wanted to leave to people who cared for her some memento behind her.

And an accident seems so contrary, so unlikely given the flamboyant nature of a character like St-Just. Or maybe, just maybe, it was done to show us how life has been unfair to her to the very end.

Now all the facts point in the general direction of an accident.
Life can be so undramatic, at times... :P