Hello Mossieur Patate!
Thank you for playing and thank you very VERY much for the extremely long and detailed comment with all your experiences, thoughts, criticisms and kind words!
Thank you as well for reporting the bugs you found, and I'm sorry that you encountered them, especially getting locked in the crater and the vehicle controls not working with other keyboard layouts.
Regarding the gravitation on Mars, it is in fact less than that on Earth because of the planets lower mass (according to the internet, https://en.m.wikipedia.org/wiki/Gravity_of_Mars)
There are invisible walls around the area which the player moves around in, so it should at least pose a little challenge getting past them to get to the edge of the map. In case the player falls through the ground / would get past the walls and fall off the edge of the map, they would respawn back at the Main Base along with the vehicle (definitely can cause a problem if you fall off in later parts of the game, will need to add checkpoints as you suggested).
Sorry to say, but so far, there is only one ending to the game, with no way to avoid the final shadow. I am planning on expanding the game a little bit (more reason to add the checkpoints), but I'm still very new to game development (this being my first published project, my first one being worked in since the beginning of this summer) and just started a new school year, so I'll probably at first focus on fixing bugs as soon as possible, and make bigger / more detailed changes later.
The idea of looking at the shadows to make them disappear sounds fun (maybe even something like getting rid of the dark, so the shadows can't be there?), I might try implementing that if you wouldn't mind me using your idea (I would also add you to the credits / am planning to add credits to the end in the game).
Once again, thank you for your comment, I'm very happy you enjoyed my game and took the time to give so much feedback :).
Have a great day!