As a few have pointed out, you cannot buy cards without owning diamonds/gems.
I could see a few ways this could be addressed; either have the cost reduced, quantity check ignored, or allowing of free gems shopping; clicking the store bundles, and granting them despite a failure to purchase...
Is there anything that can be done concerning the premium heroes, as they are not unlockable in the tavern recruitment system.
In the Endless tower. either the enemies have ridiculous amounts of health, which is not impossible, or perhaps the attack multiplier isn't working there; there might be...