To be fair, as important as Caemlyn's destruction may seem to us, in the grand scheme of things, in Verin's eyes, it was really kind of a side issue. Nice if it can be prevented, and worth a letter, but not crippling to the Light(well, she didn't know about the dragons, so not crippling to the light as far as she knows), and not worth a whole lot of her time. It was a mistake, but to be fair, we, as the fans, who have been inside Mat's head for a dozen books, almost all thought he would open the thing too. So it seems reasonable that she make this mistake, especially since it wasn't all that an important one.

And even though the letter didn't get opened, at least the Band would be in Caemlyn to fight the shadowspawn. She also had no way of knowing that Elayne would take half the band to Cairhein, or that Mat would be gone.

So most of the screw-up isn't really her fault. And we don't know exactly how restrictive the whole 'she can't betray the DO' is, so I am willing to give her the benefit of the doubt for now.