Hmm not sure if she is really dead, I don't think the author would throw an unresolved point of her thinking that it was Seo who sabotaged her plan and then just end her like that
It will eventually come up if Seo is only pretending to be amnesiac. Think about this possibility: Seo was the one who actually swapped the oil and had Michiru try to set Miho on fire. Prez and vice-prez had nothing to do with it and thought Michiru herself tried to kill Miho, which "justifies" them causing Michiru's death much more than if they were the ones responsible for swapping the oil. It would also cause them to have to come to terms with having mistakenly killed someone.
Problem is that Stuco had mpre reasons to swap the oil then Seo, i mean come on, they try to kill Miho multiples times now. And even in the tiny possibility it's true, you think they wanna to get rid of someone who can potentially help them kill Miho ? Doesn't seem right.