So — well-designed or not?
To answer the opening question: yes. Every pair of cards in the printed deck has a stable solution, in any rotation, under the actual game's rule that piece-skipping is allowed. The deck is a clique in the compatibility graph, with no missing edges. The 12 -canonical shapes were not picked at random — they form a genuine subset of the 102 possible silhouettes such that every pair plays.
The deck could be extended: and can be added without breaking anything. Several other extensions (, ) introduce marginal-only solutions, and adding both produces an outright impossible pair. Section 7's compatibility graphs let you build candidate decks by hand and see which ones survive.
Project Cube is © Alain Rivollet, all rights reserved. This post is an analysis, not a substitute. The fun bit of the puzzle is the head-to-head race against someone else — sitting across from a person staring at the same two cards, both finding a particular puzzle way more difficult than the other. That experience is in the box, not on this page.