invented to keep out the cold
The truly crucial piece here is to direct attention away from her information source. The strongest blade is the hidden blade, and if Ulon has showed something like prophecy now, he still wants it kept secret as much as possible. Having prophecy available is a strength, but having prophecy available to you without any others knowing of it is a far greater one. Similarly, it being known that you have prophecy is a weakness, though less of one if you in fact do not.
Her job here is therefore to misplace suspicion for who has prophecy here. She can't deny the existence of prophecy, because her previous letters need to demonstrate it to get the conspiracy's attention, and they won't believe it. Who could have it and work like this? There are several options, but one of them is a prominent foe of Ulon's cult, and could do with some inconvenience.
Will the gods actually believe this? Norgorber is known not to answer Communes truthfully, and to somehow break laws and get away with it. Anything which verifies a lack of prophecy would... maybe... also be able to verify a lack of other crimes? Forcing Norgorber to accept that would be a victory. Maybe. She doesn't actually know how any of this works, but it seems like it should be. She doesn't actually know how gods think. Still, this seems like a plausible story which it will be hard for anyone to disprove, and hopefully that is as true of gods as it is of mortals.
And, of course, this all needs to come from someone who believes the story she's telling, because if it came directly from a source which knew what was actually happening she has no doubt a god could see that. Fortunately, a foe who knows you is an obstacle to overcome, but a foe who knows you not is a tool for you to wield. Convincing the conspirators of Norgorber's presence should be doable with careful wording and only some minor redesign of the plan to play up his aspects. It's not like she was that far off anyway.