Obviously - there are many ways for events to unfold, just look at the Mirror Worlds.

I actually think the Rhuidean ter'angreal could always show the future. There's some quote back in tSR or somewhere that a man can go to Rhuidean once, a woman twice, and more means death. Since the women are going through the rings first of all, that means it's going through the columns for a second time that kills both they and the men. Why? Because the truth they face the first time - the past - is harrowing and horrific, but within their paradigm. It can be accepted. The future, though, was too alien, too inexplicable. What Aiel apprentice, before Aviendha, would even have been able to understand what they were seeing?

I read that scene as a warning that the Seanchan have to be dealt with by Rand, from a position of strength. I sort of assumed that they would keep their lands, and the other nations would keep theirs, and that would be that. But that's just naive. The Seanchan are aggressively expansionist conquerors, and very good at what they do. If they aren't stopped from leashing channelers, they'll win, pure and simple. They can't be ignored. And Rand is the only one who can do it. Perhaps that's Aviendha's purpose in the Pattern - to take him that warning.