Forcing the player to go back to level one and earn three stars in order to make it to level four was a genius bit of design. It reinforces to them that the mirror will spawn on the opposite edge and makes sure they understand a core mechanic before moving on. I thought that was really well implemented.
For a first attempt at music, it's not bad at all, but as you noted is too repetitive during a long play session and the good thing is this is a game you want to play all the way through. I had a blast working out the puzzles. The art is simple, but it's cohesive, so it makes the whole thing charming.
Really well done for a small team. Great job.