Thank you very much for the detailed feedback and the great suggestions. The movement (logic) is definitely on my list of changes when I will revisit the project (after the positive feedback I am actively considering it).
The alternate control scheme (WASD) was added in the beginning, then axed/ignored during development (that is why I did not show it in the tutorial, since it was not feature complete). The "E" key for interacting would make a lot of sense in this control scheme. "P" was chosen because of "pick up objects" long before the other interactions were thought of. Additionally, on the laptop I developed the game on, the "P" key was in comfortable range of the arrow keys ...
Originally, the level was planned a lot larger and the secret areas would have been hidden/disguised, probably behind portals/worm holes that the player would have to activate at first to even see the areas. Due to time limitations, I had to scrap this idea as well as the second vertical level of the map (where even more surprises would have been hidden).
Great to hear that you enjoyed the game nevertheless.