2) what kind of animal is drawn on the right wall?
It used to be the "bunny," but Mateusz altered it, so perhaps it's a caveman. After all, there are other cave paintings.
3) is it a cave on lower screen? Why we can't go there?
5) What is above "black area"? Is it part of mountain? Why we can't move over it?
3) (as Vortex said)
5) It's a chasm seperating us and that part of the landscape. It only
looks like the mountain is on both sides, but I believe the dark space is some part of another mountain across a chasm.
And besides, Mateusz would need to add a "forward" direction (like entering rooms) in order to go there, and that would break the whole "Layer scheme" that Mateusz was showing us. It's like asking why we couldn't go above the ladder in Layer 6.