So until the save game (which I still think may be a fake out) my theory was the older brother is dead and the only reason the rich parents had a second child was that he had a terminal disease and they were trying to have a child in hopes of having a match for donation of some sort. The switch...
And confirmed Oni san is under 30. Still creepy age difference but not as bad as when folks were assuming he was in his forties (pretty sure they were mixing him up with Kohei-chan's Senpei).
You're mixing up your Girls. Ai chan is dateing salarykun and yes its heavily implied that they are sleeping together, but she isn't preggers in the Pre-serialization. Its the Student Council president who ends up preggers with the Sensei that she spent 3 years pursuing, but their relationship...