I agree with you that Graendal's survival seems pointless.

But her plan wasn't that stupid. Read again the part where she asks Slayer to spring the trap anyway. She mentions yet another asset that she has. She talks about Perrin's ta'veren nature and how that might deflect even arrows.

Then, after the plan fails, she curses one of the Whitecloaks.

So what almost happened to Perrin during the chaos of the battle that involved Whitecloaks? Why, Byar attempting to kill him from behind, of course!

In the chaos of the battle, there was nothing Perrin could have done. He was taken totally by surprise. If not for Dain Bornhald (likely the "cursed Whitecloak", Perrin would have died, just as Graendal planned.

That the asset is Byar also fits the fact that he was near Arad Doman early in the series. We always assumed his talk of Perrin killing Geofram was his typical Whitecloak attitude of guilt before proof. But doesn't it seem like the kind of cubtle compulsion Graendal would use, and the way she uses him fits her MO of having multiple plans in motion all the time.

But they could quite easily have given this role to Moghedien. Graendal being quick enough and clever enough to escape Rand just become yet another of SH's rape victims seems absurd to me. Lets hope she has a role to play in aMoL.