I'm not going to deny you are technically right but this was a point that was brought up in the story - Tuon had promised Beslan she would secure the rest of the country, since the rulership had effectively been reduced to Ebou Dar and immediate surrounding lands before then.

But I don't think borders matter if you can't exercise control over them. If you can't do that, borders are nothing more than lines on a map, even if the noble sitting in the well-defended capital hundreds of miles away throws a fit. And a war at this point against the Seanchan with a fully united Western effort destroying the Seanchan occupying the Westlands now might actually be the best time to strike, given that the Empire is in a shambles and the Return has been cut off. Dark Rand probably could have done it himself by eliminating all of the Blood at once. Unfortunately Tarmon Gaidon makes this impossible, with so many enemies literally at their backs and poised to besiege all of their cities.