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.

If the target creature becomes an illegal target, the Equipment remains on the battlefield unattached.

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.

Attaching an Equipment with its enters-the-battlefield triggered ability isn't the same as using its equip ability. You don't pay mana for the attachment, and if the Equipment enters at a time you couldn't cast a sorcery, you can still attach it to a creature you control.