1. Great music! Very cute graphics! I like the minimal use of color. I beat the stage and enjoyed the mechanics.
2. Depending on exactly what you're going for, I think it's a solid game concept as-is. It reminds me of old puzzle games like Solomon's Key or Adventures of Lolo. If you want to do something with a bit more depth, you could add a bit of story, but that's up to you.
3. I like the challenge level and pacing. It might be nice to have an in-game explanation of the controls.
4. This is up to you, but I think it would be a fun coding challenge!