Nope, you've found everything. 0-stone = "nullstone", but I guess I didn't actually put that in the game's writing anywhere! Oops.
The third difficulty is something I plan to add in the near future, not in the game yet.
As for the winning deck against battle 1, it might be because you lost the initial tutorial fight, and then came back and beat it much later? Or it's a glitch. I have absolutely no idea, I guess a random unused sprite somehow got loaded in-place of X-stone or something??? The Unity Resources folder is weird sometimes I guess.