Or she did know, and was unable to do anything about it.


It hastened the battle that gave her the throne. The approach of the Borderlanders galvanized the old-fart faction and made them start approaching Arymilla about putting aside the War of Succession in order to unite against the invaders. As was reported in KoD, shortly after they communicated this intent to her, Arymilla's forces stepped up the tempo of her attack on the city, trying to end the siege before the others could come and force her to stop the war. In such a situation, their force being the largest mobilized army active, and an apparently united front (as opposed to Arymilla's which consisted mainly of dupes, hirelings and captives) they'd dominate whatever alliance was made to fight the Borderlanders, and one way or another, end up putting their own candidate on the throne at some point, who would almost certainly NOT be Arymilla.

Because the Borderlanders' "invasion" stirred the recalcitrant clique to action, Arymilla had to beat Elayne before they arrived at Caemlyn. That led to the information about her plans being leaked (she had to show her confidence in ultimate victory, what with so many other factors converging) and Elayne being able to use that to capture her. Without the other nobles getting involved, she might have been content to wait the siege out a while longer, and not giving Elayne the chance to capture her and her supporters before the old timers finally DID show up to stick their oars in. As it was, if Arymilla had been able to win that final battle, she'd have been in pretty much the same position as Elayne was when the others did arrive - possessing a plurality of support, the captial city and no way for another candidate to assemble the requisite number of Houses.

Taking the city before the others came forced a decision in the Elayne-Arymilla showdown, and took the choice of queen out of the hands of the faction most likely to resist necessary changes and drag their feet on any non-traditional measures taken to deal with the Seachan/Shadow threats. And the decision was forced because the Borderlanders' approach forced everyone but Elayne to speed up their timetables.

A recurring theme in WoT is the necessity for the good guys to stir up the pot or take an audacious gamble when the situation or status quo is stacked against them. This is how Elayne is able to win - because with the smallest army, the weakest strategic position, and with the most hands turned specifically against her personally, she would have been doomed in the normal course of things. It's WoT in a microcosm - the good guys are doomed against the power of the Shadow, which is why they need game-changers, like the Dragon Reborn & two ta'veren, the Horn of Valere, the Cleansing and so on.