Hi Olaf,
Thanks for this long and meaningfull feedback <3 and sorry for the long time it took me to reply.
The difficulty level is the thing that I was worried about that if I make it too hard people will just bounce back after few levels, but from few feedbacks including yours I see that I should make it at least a little bit harder.
The on screen button issue I will add to the todo list, It was there because I was developing it few years ago with idea of making it mobile only, but right now I think I will start with demo for web and then Steam(I'm a little bit scared of oversaturated mobile app store, where for me Its Very Very hard to find a good quality game).
Adding Two-Character mode into the main game mode is also a good idea, It can add another level to the puzzles.
"It should be more self-explanatory, though, so I don't have to read how the level works" This was to see which worked better, an on screen button for changing characters or an in-game button that you need to click with your character.
"if the other blue box looks more like a closed door or a set or iron bars or something, it will be immediately intuitive that "I have to find a way to open that - maybe I should press that button over there and see what happens"" Another one landing on my ToDo list :P
For the limited moves/time challenge levels, I will test that, after adding all other things.
Anyway, that was a Very helpful feedback, thanks to You I have a few more aspects to focus on when developing further the game :D