Thanks for you detailed feedback! It's nice to hear that at least someone enjoyed ghosts as many have commented that waiting for the ghosts was not very fun. About your other points:
Yes, the current levels are relatively "easy", meaning that they do not necessarily need more than one ghost. The player might still get killed accidently so I wanted to have the levels scale easier if necessary with the help of the second ghost. Having only one ghost could have made many levels difficult for most players. I believe you found some of the shortcuts in the levels: the normal solution for level 2 needs 1 ghost and level 3 needs all 3 ghosts. But as the game is also a platformer I wanted to give a possibility to "cheat" the puzzles with accurate platforming so if I'm correctly guessing what you did to solve those levels, they were left there intentionally (even though the trick in puzzle 2 was just an accident at first).
There has been also other comments about jumping so I have now fixed most jumping related issues that with some help from Vendolis.
Your suggestion for the ending would have been cool. I had similar idea at one point but then I decided against because I was worried that having the game both go black and end the music could be seen as a glitch. If I would improve the game I would want to make the ending more dramatic: maybe having the music cut at first and then after a while proceed to an ending screen or main menu with a melancholic version of the main theme playing. If I were to make a full game out of this there would probably be even an cutscene at the end.