Errrrrr, while I don't have much expertise on the matter either, I disagree. Just because of the pattern of how Alex is fucking up things. It is an if-else code, it's actually kind of easy logic to follow. And primary reason why he codes like that, after all.
You can clearly see when Osana goes to the rooftop, she still does her animation of putting shoes on the shelf. This is a major clue. It seems crucial to inform Osana about the letter before she gets to the locker. Because she reads letter first, she never gets a chance to take shoes off. She was locked in "letter event". After getting to the rooftop though, she is finished with "getting to the rooftop" event and because she's at school, she's "taking off her shoes". And here, spaghetti code begins it's marvel. In third kill for example, Osana state is doubled for some reason. She's like Schrödinger cat. At the same time, she's on the roof, but she's also taking off her shoes in the entrance. Thus, you can kill her twice.
Isn't it amazing how "taking off the shoes" event happens when student cross some point at school, and at the same time, student can bypass this check through another event? This is nothing new in his game. One event, when started, will have total control of the game until it's finished.