If a split card is turned face up, the ability counters all spells with the same name as either side of that split card.

The only cards you turn face up are the cards exiled with the Grimoire Thief that was sacrificed.

The last ability counters all spells on the stack with any of the appropriate names, no matter who controls them.

Once a player gains control of Grimoire Thief, that player can look at all cards it exiled, even if those cards were exiled while someone else controlled Grimoire Thief.

Once a player is allowed to look at the exiled face-down cards, that player may continue to do so even after Grimoire Thief leaves the battlefield or another player gains control of it.