You can exit the computer by hitting the exit button in the computer's UI, but I intentionally lock the player in place during the echo loading sequence as to not cause any bugs (A quick solution to a potentially big problem).
Also, the reason that there's no multi-note polyphony is because clients would run into performance issues about 3x quicker. I would need to have 3 audio sources on a player instead of just once, which would not scale very well if you have upwards of 100 echoes.
Thank you for the feedback though! If I can find a way to get multi-note polyphony without destroying the game's framerate down the line, I totally will!