
Since we're really getting into it now, here's how I see prescience in Dune as a purely down-to-earth, scientific non-mystical-or-"something extra" thing.

There seems to be four factors that determine how well someone can predict the future:
1) Ability to make mental computations. This is something that both mentats and the guild seers get a lot of training in. Mentats do basically make accurate predictions about the future, though you wouldn't call them prescient yet. It can be a subconscious process.
2) Access to data. The best ones at this are undoubtedly Reverend Mothers. Not only are they able to take in minute details that others would miss thanks to their training, but they have access to the wealth of knowledge that is Other Memory. A Kwisatz Haderach would be even better since they get even more data.
3) Breeding. Your genes are important and hold the key to certain factors in the process: things like sensitivity to drugs certainly, chance of surviving agony rituals, maybe "potential" but I don't think that's entirely made clear. The BG practice this and, I thought, so do the Guild... Though KJA/BH don't seem to think so.
4) The right drugs. And lots of them. This is kind of linked to number 2, in that the "awareness spectrum narcotics" described in the books open your awareness up to a plethora of new data for analysis. It's how the fremen feel the Tao in their orgy: their awareness is more receptive to things they would normally miss and so they feel connected. The best drugs are the Spice/WoL perhaps followed by the truthsayer drug.
When you add up all these things you've basically got an oracle. Prescience is not a magical gift from the gods given to the chosen one, rather it was expected, calculated. The Bene Gesserit assumed that the Kwisatz Haderach would have prescient abilities and would be something like a Guild navigator. What they didn't anticipate was the level of ability that the KH gained from other, environmental and nurturing factors.
There does indeed seem to be a line of ability where you would say that this character is prescient and this character is not, but it's not entirely clear where it is. But what does seem to be clear is that everyone who can predict the future does so in a mathematical, calculated way... subconsciously or not.
As for prescients not seeing other prescients ...


Remember in the first Star Wars film when Obi-Wan and Darth Vader were fighting? They didn't actually do very much. At the time when I saw it I assumed it was because they could both see what was going to happen so it was a stalemate, if they tried to do anything fancy there would be a massive game of mental chess going on before any lightsaber action happened.
So yeah, sounds silly right? But what if you have a similar situation with Dune prescience? You get two prescients looking for the actions of the other, every time they look, they spawn a potential future which the other one can see, which spawns another potential future which the other can see... and you get an infinite feedback loop.
I personally think that prescient invisibility can be explained because of this (not only a plot device

Back to Mohiam. In my opinion, it's entirely possible for her to have enough of all of those factors to have prescient visions - vague ones because they're subconscious like Paul's dreams. Though she's certainly not as powerful as a Guild seer.
Err. That turned into quite an essay. Hope it all makes sense

