Soaring Thought-Thief’s fourth ability causes each opponent to mill two cards, not just those being attacked by Rogues you control.

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.

Soaring Thought-Thief’s third ability applies to itself as well as other Rogues you control.

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.