I'm mainly posting here because I have found 3 different solutions for the last level (SPOILERS ahead) and I am not sure they are all intended:
1) Throwing the magnet onto the switch (by switching polarity mid-air) and jumping above the pit before the [-] platform comes up 
2) Throwing the magnet next to the bottom switch so that the blocks doesn't go all the way down and doesn't close the way to the key
3) Using the door to prevent the block from going back up (which I guess is the intended solution)
Also I wanted to say that these videos are fantastic, great work ! The game has definitively improved since last time and it feels really good to play now.