400.7. An object that moves from one zone to another becomes a new object with no memory of, or relation to, its previous existence. There are seven exceptions to this rule: [...]
Another way of saying this is:
If an ability knows of an object, and that object is moved, the ability henceforth considers that object to have ceased to exist.
Now let's take a look at Act on Impulse. This Sorcery's ability is
Exile the top three cards of your library. Until end of turn, you may play cards exiled this way.
For Act on Impulse to let you cast the cards in exile, it must know about them (or else it would never work). According to CR 400.7, the cards exiled by Act on Impulse are brand new objects in exile unrelated to the objects that previously existed in the library. So how does Act on Impulse know of the cards in exile? It's not one of the exceptions of CR 400.7. Act on Impulse can see the cards in exile because it's the one that created those objects.
Something similar is happening here. For Karn Liberated's ultimate to be able to put the exiled cards on the battlefield, it must know about them (or else it would never work). So how does Karn's ultimate know of the cards in exile? Karn Liberated's ultimate can see the cards in exile because it's linked with the abilities that created those objects[CR 607.2a].
So, your question comes down to two simple questions:
Does Karn's ultimate on the stack continue to exist after Karn ceases to exist?
Yes[CR 112.7a]. This is explicitly spelled out in the rules.
Does Karn's ultimate see the objects exiled by Karn's other abilities?
Yes[CR 607.2a] (assuming they never left exile[CR 400.7]). If it didn't, it would never work.
Therefore, Karn's ultimate is able to bring back Emrakul even if Karn ceases to exist before the ability resolves.
As you can see, CR 400.7 doesn't even come into play in your scenarioes. CR 400.7 would come into play if Living Wish was used to remove Emrakul from exile. If someone did that, Karn's ultimate would not be able to return Emrakul to the battlefield because the object created in exile by the linked ability no longer exists.
I hope this clears up your confusion.
112.7a Once activated or triggered, an ability exists on the stack independently of its source. [...]
607.2a If an object has an activated or triggered ability printed on it that instructs a player to exile one or more cards and an ability printed on it that refers either to “the exiled cards” or to cards “exiled with [this object],” these abilities are linked. The second ability refers only to cards in the exile zone that were put there as a result of an instruction to exile them in the first ability.