Evidently, you were incorrect. As far as not holding northern Altara, that is irrelevant, it is still nominally part of Altara. For comparison's sake, Andor only nominally holds anything west of Whitebridge, which does not stop Andor from treating the raising of a lord in the Two Rivers as an actual rebellion. Borders actually do matter. If Andor or Murandy or Ghealdan tried to seize that area, that might not only give the monarchy the outside threat it needs to extend its influence over its own country, but it would also draw the attention of those nations' enemies who would rush to defend Altara to stymie the expansion of those countries.

If Andor were to cross Altara's borders to "defend" the road against the Seanchan, then that would be an invasion of Altara and an act of war against the Seanchan. And probably futile - as mentioned earlier, Andor itself hardly holds its own territory west of the Arinelle, and most of the Altaran territory being discussed is also west of that river.