The way Herc's card states, it's a must if able. If there is a heartless that you can bounce within the level of Herc's effect, then yes, you must bounce it. If there are more, then you must bounce one. If not, then Herc's just a friend card that comes into play with no effect.
'N no, you cannot play dark cards on your own world card. Otherwise, I'd have the ultimate deck for sure. The dark 'n nobody cards are solely to use to hinder your opponent. Otherwise, a player could play only level 1 cards 'n a simple Shadow for their own worlds to ensure your opponent can't play any cards on the world. It is unfortunate you can't 'hinder' yourself for certain goals. With my deck, playin on my world would prove me unstoppable!
But not for the reason mention'd above. A different reason altogether. Mwahahahaha!!