Regarding who can see the Whispers: perhaps one has to come in contact with
both the Whispers and someone who can already see them to gain the ability? Maybe the contact even has to be simultaneous?
One of those could explain why Yuffie, Reno, and Rude can't see them yet ... Though option 2 would still leave us to ponder when precisely Jessie, Wedge, or Rufus would have come into simultaneous contact with Cloud
and a Whisper.
Personally, whatever the explanation is, I feel there needs to be one, as I find it utterly implausible that all three (Yuffie, Reno, and Rude) aren't considered to be in a significant enough position to influence the future. Yuffie's a main cast member for goodness' sake, and the Turks literally prevent Sephiroth's early resurrection in "The Kids Are Alright" -- to say nothing of their contributions in AC!
Since the Whispers can be viewed as entities existing in multiple dimensions of time, though the Whispers are still limited by how many of them are around and present for duty, I'm assuming the Whispers saw a potential future unfolding where the plate didn't drop. Upon seeing the future getting remolded, the Whispers took action in the present and ensured that the plate drop happened.
This makes me think of what we saw happen in the Singularity, where the Whispers made a copy of Midgar, and it was then that copy which was destroyed amidst imagery of "Aerith's world of the Lifestream" and then "Sephiroth's world of Meteor."
Perhaps the Singularity is, for the Whispers, like a test website where they take a "copy" of the real-world, dump the "data" into a test space, then run simulations to determine their next moves.
Never was there a fictional device of which it more needed to be said, "Don't overthink this."
Yet here I go
If the Whispers can exist in multiple dimensions of time, why can't they simultaneously be dealing with everyone at once?
Given that they run reconnaissance and have to consider steps, they're not omniscient. In-the-know, definitely, but not all-knowing.
Lic said:
They could repeatedly return over and over and over again to different physical locations at the same time coordinates. Even if there was only one Whisper, it could do this -
a bit like that scene from X Men Apocalypse where Quicksilver saves everyone. To a bystander moving at normal human speed, it would look as if there were multiple Quicksilvers, possibly one for each person rescued.
I think, as Mako suggested, they have access to all of time, but once they have selected a particular instant as their entry point and played out an event, they're either locked out of that moment or can't trod back over their own tracks without interfering with their personal timeline. A la "Doctor Who."