It seems very unlikely that the Borderland army is under direct control of the Shadow (i.e. I doubt Demandred or another Forsaken or even a high-ranking Darkfriend is there giving orders). The POV we got when they set off made it look like the rulers at least thought it was their own idea.

But it's still very possible that the shadow somehow manipulated them into taking that action. There's no evidence of such manipulation that I'm aware of (other than the possiblity that most or all of the AS with them are BA), but it's just too fortuitous for the shadow to be coincidence. Then again, it's possible that the Borderlanders being where they are may turn out to be a stroke of luck for the light... but I doubt it.

I'm desperately hoping that some kind of major revelation happens regarding the Borderlander motivations, because their stated objectives and reasoning are extremely weak. If they're not being manipulated by the shadow or pursuing some kind of meaningful hidden agenda, then RJ has seriously character-derailed four potentially cool characters (or more than four, if you count the senior advisors).