There is a key truth about Nethys which one could deduce a priori, if one knew an a-priori truth that is not common a-priori knowledge in Golarion.

That key a-priori truth is as follows:  Everything that's real is causally well-founded.

The key definition:  A well-founded ordering is one where there's no infinite descending sequences.

The natural numbers are well-founded; no matter how high the number you start with, if you keep going downward, you'll eventually reach zero and then be able to go no further downward, no matter how clever the path you choose.

The negative integers are no more numerous than the positive integers, but their ordering is not well-founded; you can go down from -1 to -2, down from -2 to -19, and so on down forever.

Similarly, it happens to be an a priori truth that all consciousnesses find themselves inside of a continuum that is made up of quantities of realityfluid assigned to identity-points related by directed cause-and-effect laws (eg quantum amplitudes, in dath ilan).  And all these continuums of causal relation are causally well-founded; they contain no infinite sequences of causes of causes going forever backwards in time. 

Different continua have different Big Bangs—different degenerate-zero-points in their relations between the stuff of realness.  But they all have some equivalent of a Big Bang.

Some continuums last forever, they causally increase in time forever—though they necessarily become less and less real as they go, for no stretch of realness can sum to being infinitely real.  But a cause-of-effect sequence never goes back forever.

As a special case of this rule, time never goes in a loop.  Because then again one could talk of a cause, and a cause's cause, and a cause's cause's cause, and so on forever, even if you were covering the same territory and repeating yourself.  If you say that A > B > C > A... then that's not a well-ordered set even if it's a finite one.

(And why is that so?  Why is it that when you look about yourself—literally no matter who you are, anywhere, everytime—you find yourself inside a universe with a finite past?  You can, if your sight as a god or a civilization stretches far enough, see back to the local Beginning and know that you are in fact within a continuum with a finite past.  But why?  Can't logic describe possible laws of physics that include consistent time travel, or universes with infinite pasts, just like math can describe an infinite descent through negative integers?

And while these margins are too small to contain a satisfying answer, one key point is that local relations of physics can never pinpoint an event with an infinite past; all first-order axiom-sets with infinite models have models of every infinite cardinality, a vastness of unconstrained possibility whose numerosity is literally indescribable.  So while time can go on forever forward, the real people always find themselves at some particular finite time, because otherwise you couldn't be found at all.)

From Causality's well-foundedness, it follows that even when 'prophecy' was unshattered, gods could not see the future.  For to see a thing, is to be affected by it; to see a thing, is to have that thing be the cause of your observation; if X sees Y then Y is an ancestral cause of X.  If the past can see the future, then the future has affected the past, and a cause would have a cause and a cause's cause in an ill-founded causal ordering that descended forever.

And simple empirical observation confirms that prophecy in Creation never spoke of the future, for then it'd have simply been true, and unshatterable.

A priori, then, if you know which sort of continua can be real in the first place, you know that Nethys cannot see the future, God of Knowledge or not.

Then what is Nethys seeing?  Mere possibilities that aren't real?

Most civilizations - unless they are in some very confused stage of learning about their equivalent of quantum mechanics - will know better than to utter anything this confused: you cannot see things that are not real.  Even when you hallucinate you are seeing a real hallucination, there is something actually happening inside your brain that makes you see it.

To see something is to be affected by it; and real effects cannot be produced by unreal causes.  If the world-history of a photon going down a particular pathway affects your experimental results, then that world-history must have happened as much as anything ever happens; if all possible combinations of qubits contribute to a quantum computer's actual output, then all possible combinations of qubits must have been 'real'.  Only real causes have real effects; so whatever affects a real thing must also be real.  Causes and effects don't have be equally real - you can set things up so that a more real observer can see the output of a less real process - but the cause must be at least a little real.

(Though these margins are, again, too small to contain that logic whereby 'realness' for the purpose of 'the conscious entities described inside a causal system can find themselves there' and 'realness' for the purpose of 'real effects have real causes' are necessarily the same kind of 'realness'.  Without that connective step, the above is just a bare appeal to philosophical intuition - but a philosophical intuition that happens to be correct.)

Now consider: while Nethys was not perfectly accurate in His forecasts of what would happen around Keltham, He was able to pull some fairly intricate shenanigans, on the order of sparking off the Zon-Kuthon godwar and furthermore arranging for Pilar to be in place to take a sword for Keltham.

The 'possibilities' that Nethys glimpses are evidently detailed enough that they must have been less like abstract imagination-guesses, and more like full-blown causal universes themselves, very similar to the real Golarion.

And now comes the well-foundedness issue:  If Nethys sees a 'possibility' including a Kuthite sword aimed at Keltham, this 'possibility' evidently contains its own Nethys, without whom a Zon-Kuthon godwar doesn't get set off in the first place.

Does the Nethys inside this 'possibility' also see a great field of secondary 'possibilities', each such secondary 'possibility' containing its own Nethys, who in turn sees many tertiary 'possibilities' containing Nethysi, and so on forever?

And the a priori truth says: no, that can't go on forever, not if Nethys and Creation and the Magical Continuum exist at all and have consciousnesses finding themselves inside them.  You can't have a setup in which Nethys sees other possible Golarions, with those Golarions containing Nethysi (as Golarions always do) and all those Nethysi see Golarions containing other Nethysi and so on forever.

To see a thing is to be affected by it; "Nethys saw an alternate Nethys who saw an alternate Nethys who saw..." is ill-founded causality, infinitely descending through causes of causes.

Consider Keltham materializing into Golarion, near the Worldwound.

Consider the notion of Nethys looking around this event, and seeing how that event went in the futures of Keltham-containing possible Golarions with their own meddling Nethysi; and therefore becoming an effect of whom those other Keltham-containing Golarions and Nethysi were a cause.

Then - if you trace back and back in the causality - you must eventually come to a Nethys who sees Keltham at the worldwound, and has not seen the futures of any possibilities containing a Keltham.

And the same logic applies wending through time: at the event-point "one minute after the arrival of Keltham into a Golarion with a Nethys who has not seen any other Keltham's futures", there must again be some subsequent Nethys who has not seen any later future of a Golarion like that.  Even if they are relatively very rare among Nethysi, they must exist in order to provide a causal foundation for all the towers of observing Nethys built on top of them.

Which is all to say - if Time and Causality are well-founded, and in fact, they are -

Among the possibilities that it is possible for a Nethys to foresee -

There must be a possibility containing a Nethys who foresaw nothing at any point of how it would go with Keltham.

It begins with Keltham materializing at the Worldwound, and running into the building closest to him, containing Carissa Sevar.

The first steps play out in just the same way.  Nethys doesn't have a startled reaction to somebody materializing whom He has never seen in any future possibility; Nethys is fragmented and the fragments take time to communicate, and 'no answer found' replies are harder to verify than 'answer found' ones.

The fragment of Nethys that sees Keltham wonders what other Nethys fragments know about Keltham, and casts glances towards Nethys-fragments in alternate universes at later clock times, searching for a piece of Himself that knows more.  It will take some time for the Nethys-fragment to reach the surprising conclusion that His greater self spread across possibilities seems to know nothing; such that this Nethys is witness to either a truly unique one-time event, or perhaps is one of those rare Nethysi who form a base of His own well-founded vision.

And then later, having probably-verified this delightful surprising fact, Nethys will still do nothing.  Action is painful to Him, it is painful to pull enough of Himself together to act.  Nethys does not superintelligently forecast the future but observe alternate forms of it, and His shattered mad fragments are not able to deduce in advance how it will all go, only make obvious guesses like that Golarion will be swifter to invent magical nukes.

That zeroth Nethys will guess that matters will play out in an already-interesting way, which the zeroth Nethys does not yet see a vast stake in disturbing.

Boy meets girl.

Girl reads boy's mind.

Keltham envisions Abadar's special thing from scratch.

Abadar bargains with Asmodeus to ensure that this strange being eventually reaches Osirion in condition to have teachings bought from him.

The priest at the Worldwound receives Asmodeus's vision.

Project Pet Outsider is begun, in an archduke's villa.

Eleven girls among the current graduating class in Ostenso are sent to Keltham as a welcoming present.

...and later that night, Keltham looks towards Abadar again, and Abadar grants him three cleric levels (or as they say among the mortals, two cleric circles); for Nethys does not, in this possibility, offer to help pay.

Keltham's hosts go on reading his mind.  It might not be safe for Carissa Sevar to do it, anymore (and she sure is not happy about that), but it is safe for fifth-circle Securities and above.

Carissa Sevar comes to Irori's attention as before, and Irori bargains with Asmodeus...

But Ione Sala is not oracled of Nethys.