The monsters focusing on the place, could be the tamer from the start that got hired again?
I have strong doubts about that arc big bad truly being dead, we don't see the death, we don't see the corpse, a body double for a noble and him going into hiding wouldn't be anything strange for this kind of story.
Now someone could be wondering why they didn't just kill the kids, simple, they want a intenational incident and the fault of it must fall on the right faction, every information that would reveal who selected the mountain for the exam will be scrubbed or it was on all placed on a scapegoat, MC and the noble troops and mercenaries failing will make the fault go on the other side, the kids were probably more able than expected or the attacks were organized to tire them out but don't kill them before the MC arrived.