You could avoid the "cheat" issue by detecting reachable squares from the last placed and enforcing that. I also agree that the colors are too subtle and cause mistakes to be made.
I like the auto fire personally. Sure, an option to pound a key/button to death may be OK, but without ammo to worry about I would just hold it down anyway.
I don't think "A simple, soothing flash game" is correct: the difficulty level is high enough to make the Pope swear. The lack of even a hint of the "real" cursor combined with putting notes out of reach at the beginning (it only goes high enough to catch it if it is near the middle) and going off screen when you get too slow or fast? Bah... the only thing that kept me trying it was the hope of hearing more of the music... which will never happen.