Since the Prophets see to exist out of linear time presumably they experience their entire lives simultaneously.
So how long do their lives seem to be to them subjectively? If the Prophets don't experience linear time that question seems meaningless.
So how long are the lives of the Prophets in linear time? When, in linear time experienced by everyone else did they come into existence and when will they stop existing? Nobody knows.
Do the Prophets, like many other intelligent beings, have any way of knowing what happened before they existed, and calculating the probabilities of events after they stop existing? Nobody knows.
How far have the Prophets traveled, and do they have, like many other intelligent beings, knowledge about events in places they never travel to? Nobody knows.
Do the Prophets know anything about events in the galaxy M87 at the center of the Virgo Cluster 55 million light years from Earth? Do the Prophets know anything about events in galaxies three hundred million light years beyond the Great Attractor? Nobody knows.
So nobody knows the limits, if any, of the knowledge of the Prophets in time and space.
Are the Prophets able to correctly utilize what knowledge they have to make decisions and give correct answers? Not all intelligent beings always use their information correctly and logically. Nobody knows.
Thus the answer to: "Are the Prophets omniscient?" can only be nobody knows.