Really cool concept. I watched your play trough video to understand how to finish the 3rd level. It seems important to jump forward in 3d mode before continuing in 2d.
I guess you keep your same 3d-depth once you enter 2d. Unfortunately this is not always clear, leading to you falling of things you thought you stand on.
I think it can be quite difficult to get the transition between 2d/3d properly but it was a very nice experiment to play.