Thank you for the explanation. Turns out, I did it with the library in reverse order, scaling the wall after climbing the scaffolding. That's why I couldn't understand why the key on top is located on a protruding ramp.
It's really cool there is more than one way to enter the library. Especially impressive for a gamejam project.
UPD: in latest build (1.0.4) jumping is indeed much more pleasant, and fall damage feels consistent now. However, it's really hard to hit anything by throwing, so much, that I still resorted to abuse to get the white key. I jumped on a shelf near it, and just picked up the books from below the key one by one, until it fell to the floor. And if I didn't know where keys are, it would by nigh impossible to notice them with these graphics.