If no world contains a timeless cause then no world contains a timeless cause.
Right.
Not quite.
Let's grant that there is a necessary being N and that N exists timelessly in all possible worlds. Let's also grant the KCA ...
P1. For any X, if X began to exist then there exists some cause C, such that C caused X.
P2. The universe began to exist.
C. Therefore, there exists some cause C, such that C caused the universe.
So now we have a necessary and timeless being N and a cause of the universe C. These are both granted. Now we have to demonstrate that they are the same being (eg, that N = C) in worlds where universes are caused. But, there is a problem ..
P1. For any Y, if Y has a timeless cause then Y is a timeless effect.
P2. The universe is a temporal effect.
C. Therefore, the universe has a temporal cause.
So given that P1 and P2 are true (and Craig grants P1 which is why he has God becoming temporal with a universe ... if P1 wasn't true he wouldn't need to) we now know that C is temporal. So we have a necessary and timeless being N and a temporal being C. Given that N is timeless and C is temporal we can no longer arbitrarily declare that they are the same being. But we need them to be the same entity (C is N as a consequence of N undergoing a change) otherwise premise 1 of Kalam is false.
But there is another problem. Given the second argument, it is necessarily the case that the cause of the universe is temporal which entails it is necessarily the case that the cause of the universe isn't timeless. But this means that although N exists timelessly in all possible worlds it is still the case that there is no possible world in which N is the cause of a universe (the cause of the universe must be temporal and N is timeless).
If there is no possible world in which an entity does X, then by definition that entity is incapable of X. For example, God is incapable of evil by virtue of the fact that he is omnibenevolent in all possible worlds. A two year old human is incapable of being father for the very reason that there is no possible world in which a two year old human fathers a child. So, if there is no possible world in which N causes a universe then N is incapable of causing a universe. It follows from this that N can't be omnipotent ...
P1. For any X, if it is metaphysically impossible for X to cause Y then X is incapable of causing Y.
P2. It is metaphysically impossible for N to cause a universe.
C. Therefore, N is incapable of causing a universe.
P3. For any Y, if Y is incapable of causing a universe then Y is not God.
C1. Therefore, N is not God.
In order for an entity to be God, there must be a possible world where that entity causes a universe and there is no possible world in which a timeless being causes a universe.