I think by that point the hawkbats had already been sent, so word of Paul and Jessica would have spread. No small thing given the prophecy the BG had implanted in the Fremen religion. To paraphrase a certain Vorlon: the avalanche and begun and it was too late for the pebbles to vote.
As for the future, yes, it is always in motion. I think Paul's perception of prescience is compared to one's view of the land. You can see what you're standing on and your immediate surroundings and what's off in the horizon. While lies between those two extremes are hills and valleys in which events are uncertain and hidden from view. The later books also explain that the presence of other prescients like the Navigators can cause a vortex of uncertainty, which is actually what freaked them out so much. They couldn't see Paul, but they saw the nexus of growing uncertainty that surrounded him.