The reason you can go faster when going backward, was just to play with the idea of "The more knowledge you have about the game, the faster you can complete it".
So I added on purpose little odd speed difference.
I also don't understand what you mean where "There is too much friction on the ground", as I believe to have set my material to zero friction. I just set a constant velocity when walking on the group.
For the airborne control, they on purpose easily goes out of hand, and accumulate too much speed. Again, in order for someone trying to speed run, being able to abuse that.
I am indeed using a Capsule collider.
For the door, I just got to build last minute, and did not get time to fix theses stuff.
And for you being stuck, I know about this place, there is a "R" keybind that allow you to restart the level.
Thanks for you feedback, I know I have a lot to learn, and improve for my platformer control!