I quickly measured my mounted whiteboard. At the bottom edge, the maximum depth is 24mm without the shelf and 70mm with the shelf.
The tray folds up when you press against it. With the tray up, the board is approx. 30mm deep.
The frame can be opened with an Allen key and the shelf should then be able to be pulled out sideways.
I hope I was able to help.
Kind regards
G. Vonwyl