Re: Triggering / activaing special abilities in general, Grave's Martial Mastery
If you play a card for its ability, it's gone from your hand.
You can't discard the Martial Mastery for its own ability. After you play an ability, it's gone, unless the card specifically states that it's kept in the play area (like Midori's Dragon Form).
Using cards for their abiliy is different than using them for combat.
If you use the 7 in combat, the Martial Mastery doesn't fire and vice versa: if you use the Martial Mastery, you can't use the 7 in combat (again, unless the card specifially states something else, like Rook's special Blocks).