Whoever said you need a days rest between core training is dead wrong. The tissue is the same kind as that which makes up your jaw and calves. It is neigh on impossible to overtrain your core. Besides that, if you are not doing a body builder work out then your core probably gets hit every day with compound work outs. TheNegation planks are brutal, try plank rollouts
At 7.2% BF your abs should show, hell I'm usually around 15 and can see them.
I heartily accept the motto, "That government is best which governs least"; and I should like to see it acted up to more rapidly and systematically.