1. The P key is backspace as I use Colemak layout I can rebind it.
2. This is known as a “column stagger split monoblock”. It is very comfortable as your wrists sit at a better angle along with the keys following the natural differences of your fingers.
3. It’s the standard thickness of any board, about 21mm for the case then keycaps on top of that. The controller is mounted into a recessed section in the case then the keys go right on top, haven’t had any issues doing it that way yet.

If you do build one, share photos over in /r/handwiredkeyboards and/or send photos to me, I love seeing when people build the things I design.