Two days later, I released the second stage module, which gave students the ability to perform a sequence of moves by watching frames of corresponding animations, and selecting the frames in which the proper alignment of the door plane is attained.
To keep things simple, each move in the animated sequence comes with a pre-selected choice of "door/wall/ghost" planes, as well as the cube edge (axis) with which the given door plane is to be aligned. In the original activity, this stage was implemented as a Blackboard Learning Unit, composed of a sequence of six assessments. In each assessment, the student was prompted to watch the animation corresponding to a move, then submit the answer (in "fill in the blank" format) containing the correct frame number, with a tolerance of either one or two frames in each direction.
Here is a "non-Blackboard" version that emulates this functionality.