Rulings

"Hideaway N" means "When this permanent enters the battlefield, look at the top N cards of your library. Exile one of them face down and put the rest on the bottom of your library in a random order. The exiled card gains 'The player who controls the permanent that exiled this card may look at this card in the exile zone.'"
Any player who has controlled a permanent with a hideaway ability since a card was exiled with it may look at that card.
Previously, permanents with hideaway entered the battlefield tapped. This ability has been removed from the definition of hideaway. Older cards have received errata to have an additional paragraph that reads "[This permanent] enters the battlefield tapped," and they now have hideaway 4.
Hideaway now causes you to put the rest of the cards on the bottom of your library in a random order instead of any order.
It doesn't matter how the opponent was dealt damage or by whom, as long as the total damage is 7 or more. You don't specify an opponent when you activate the ability.
You'll get to play the card even if Spinerock Knoll wasn't on the battlefield at the time some or all of the 7 damage was dealt.
At the time the last ability resolves, you'll get to play the card if a player who is currently your opponent, or a player who was your opponent at the time they left the game, has been dealt 7 damage over the course of the turn.