Then I would like to give some substance to your theory. It's actually my theory, so I know that.
Isabella and Neil have the same love song. Isabella told the MC that this was her favorite song as she drove the drunk MC to her house in her car. The MC replies that he learned the song on his guitar to give his father a birthday present because it is also his favorite song. A little later, the MC returns the favor to Jill for playing the piano, which the MC liked. Jill then also mentions that it is Isabella's favorite song.
In the flashback of EP5, we learn from Isabella that she left friends behind when she went to Smith & Readme College. I strongly suspect that Isabella met James during her college years. But somehow, in the game, everyone except KRJ doesn't go to college until they're 19. But that opens up a window of 1 year where Neil could have had a relationship with Isabella.
Isabella could have met Neil during her college years, though. But that was probably before she met James and Neil fell in love with Lynette. Something comes to mind. Neil asked Luis in the Lynette flashback from EP8 if he remembered the girl from a year ago. Isabella?