Wow! Thank you so much for the detailed review! As you can see, I ran out of time, and focused on the core mechanics instead of running with the idea. I made the last level in the last hour, and I was very satisfied when I saw that I was able to implement it correctly without breaking the whole game. I was planning to add more puzzles like this, but had to face the reality, and finish the game as it was at that moment.
For the controls I had several ideas, and this fell the closest to a image/map editor. I can understand if that feels weird for some. I was thinking about leaving out the click completely, and refering only to the current mouse position. Maybe that would have been better.
I was also planning to allow multi-tile copy. That would have been a nice solution for the tiny platform issue. But limiting the movement controls to only these three keys was completely my fault, not related to the time constraint. I felt that because I like this sceme, I don't need to implement others even though it would have been just a few more characters in the code.
Thanks again for playing, for the extensive review, and for the tip for future players!