Regarding the requirements for a key: The action shouldn't be enabled for keys not meeting the requirements. (Just like most other actions are only enabled for a suitable selection of keys.) The info which keys are suitable belongs into the manual and not as wall of text into Kleopatra.

You forget that multiple OpenPGP smart cards might be plugged in. Although it's probably not likely that multiple empty cards are plugged in. (For comparison: The subkey action to move a key to a card allows the user to choose a suitable slot. I think it also offers non-empty slots, but I agree that for the "simple copy" it's better to offer only empty cards to prevent a disaster.)

Nobody uses gpgtar for S/MIME