Once a single creature has blocked Nimana Skitter-Sneak, causing it to gain menace won’t cause it to become unblocked or remove that blocking creature from combat.

An ability that offers a bonus if an opponent has eight or more cards in their graveyard won’t provide additional benefits if more than one opponent has eight or more cards in their graveyard. There are also no additional benefits no matter how many cards are in an opponent’s graveyard, as long as there are at least eight.

In a multiplayer game, once an opponent leaves the game, they won’t be considered for such an effect, no matter how many cards they had in their graveyard before leaving the game.