Build in the plane you are looking at, pivot to work on the depth you could not see, and playtest in the real engine. Levels live in this browser; use the JSON box to move them somewhere safer.
Pivoting keeps your height and your position on the hidden axis, and drops your sideways speed to zero. The world holds still while the camera swings. You cannot pivot into solid matter — the attempt is refused rather than killing you. Landing a hair outside a ledge slides you in rather than dropping you, on both the axis you can see and the one you cannot.