That multiculprit bit is something I keep wondering about as well. Well, it's obvious one person couldn't have orchestrated everything.
About her annoyance, I'm not exactly a fan of interactions with her either, but not to the point of wanting her to be rid of. Guess there had to be some character people wouldn't like even if only for the sake of balance between all the girls.
And yeah, Umiko is in fact a genius. And I'm not sure about her age as a college graduate, I think she was actually 10yo, but I don't really trust my memory. But that's the problem in this game. What you pointed out is definitely a solid argument, but it's too damn obvious. Based on that everyone would have suspected Umiko and Kasumi. And with the latest uncovered memories of the MC, at this point in game, something would have suggested their involvement. And on the subject of being too eager, what do You think about Sasha? I'd like to know your thoughts. She seems to like the MC very much, but sexually, she avoids him like a plague. Tries to be funny no matter what the circumstances, sometimes almost as if she's forcing herself to act that way. Definitely eager to help with absolutely everything, including poking into vital systems on the Endurance, despite her lack of critical knowledge and training in those matters. She was (allegedly) a goddamn flight attendant. And she's 28, so she had all the time in the world to become acquainted with and trained by the company that pulls the strings behind the scenes.