You should inform the PCs (probably through Simeon or Kiernan) that going directly to Castle Korstull would be suicide: it's surrounded by a firestorm that makes traveling to the castle impossible without special protection or preparation—which Seaquen cannot adequately provide. Taking shelter underneath a fireproofed vehicle (as a certain set of plucky gnomes do) could work, but it would be a slow, miserable march with no hope of backup or escape if things should go wrong. If the party still tries to go through with it, explain that the Lyceum will not sponsor the Wayfarers to teleport them to a nearby location, so they'll have to travel most of the way on foot.
As for going to the monastery, one of Seaquen's leaders should tell the PCs that a group of agents was already sent out to make contact with Longinus and Pilus, investigate them if possible, and secure their aid—particularly to get past the firestorm that's blocking access to Korstull. This means the party is the only set of capable hands on deck to deal with Bresk. If the party decides marches off to Eresh instead (definitely don't allow the Wayfarers just teleport them to Ostalin), then...
If the party breaks the order of books and marches off toward Ostalin (or Korstull), hopefully they choose the land route. Maybe you could explain that all of the ships are remaining docked for repairs after that unusually destructive hurricane. If they go by sea to the southern shores of Ostalin or something, then you might have to make up some kind of complications (i.e. an adventure) that occurs at sea or once they're ashore. By land, however, regardless of destination, they need to travel most of the way to Bresk before hooking left, which gives you opportunities to throw in various Book 4 hooks that can occur on that road. Try to entice the party with hints of what's going on in Dassen and imply that things will probably go hideously wrong if they don't step in.
If they still break from the order and march to Eresh, give them enough obstacles on the road for a level-up—they'll still be a level below the recommended number, but at least the gap can close a bit.