Two thoughts,
1 I've always thought Nic and Co's true purpose was taking up a place and dispensing an energy type the Walker's specifically utilize. In kind of a necronomicon/alt mirror way(taking up the mirror, is touched on in Woj about if Thomas took up a coin his demon would have the short end of the stick, they'd be trying to occupy the same space)
2 state of mind, Belief, ect. Are they(outsiders) trying to create a belief in the coming apocalypse to give it power? Like a snowball effect? Is "apocalypse" an aspect of a specific being they hope to summon or allow to enter reality itself?
Right with you Sibelis.
"State of mind" is the key phrase. I think if humanity as a whole believe (or enough of them at least) that the apocalypse is happening...then it is. They create it from their own minds.
I think this is both a magical phenomenon and a philosophical state. It's not necessarily unrelated either. I don't think the Outsiders could enter and start an apocalypse en masse without mortal's belief allowing it.
Jim has said before most magic happens in the mind. He has also shown how the physical, particularly mortal world has more weight than the spiritual in most senses (but there are a few really important exceptions such as the soul). He has often talked about how mortal choice creates the world they live in and spins off other universes (although let's not get side-tracked too much here).
To me, this is all an excellent indication that what mortals believe in, have faith in (or the lack of faith in), will shape things in seriously significant ways. If the combined mortal psychic state decides that the world is ending, it is inevitable and cannot be saved, that the dark forces have come for them - then it will be so.
This is why the Angels and the Fallen (and other perhaps) fight so hard to influence mortals. Because it's the strongest way to win. It's really the only way.