a) You may use a single cube in shared memory if you prefer or go ahead
with storing just shared edges in the shared memory area. Both are OK.

b) 3d face correspondance:
In the 2D view port the faces are positioned as follows

1  2  3

4  5  6

The origin [0][0] is always the top-left corner in the 2D view
the first coordinate [this one][0] walks along x (horizontal).

In the 3D view here is a bad ascii drawing that:
a) proves I can't draw in ascii
b) shows you the correspondance of the faces to the 3d cube

The origin for each face is marked by an 'O->' with the arrow pointing
in the X direction (Y is obviously the other direction)

        /-----------------------------------
       /|                     (back)   <-O/ |
      / |                      #3        /  |
     /  |                               /   |
    /   |         #5 (TOP)             /    |
   /O-> |                             /     |
   ------------------------------------     |
   |O->                                |O-> |
   |                                   |#2  |
   |                                   |    |
   |                                   |    |
   |                                   |    |
   |                                   |    |
   |                                   |    |
   |                    #1             |    |
   |                                   |    |
   |                                   |    |
   |                                   |    |
   |    /                              |    /
   |   /                               |   /
   |  /                                |  /
   | /                                 | /
   |/O-> (bottom) #6                   |/
   -------------------------------------

The face on the left is #4. Its origin is in the Top corner that touches
face #3 (the back face). The X axis points towards face #1.

To help clarify:

1 2 3 4 encircle the cube, 1 faces you. The Origins for 1,2,3,4 are in the
top left corner if you rotate the cube so that the face is pointing
towards you.

The top and bottom are #5, #6 respectively. If #1 faces you, the origin is
on the closest edge to #1 and the X axis points to the right with Y
pointing into the screen.

This can all be verified by writing patterns you can recognize to the
cube.

Enjoy!