I have a question about the running the Horn, particularly "Thirty-One Weeks".
** spoiler omitted **
Hexor and Vexor only show up if they're commanded to by the villains, should they know that there's issues. Each group that comes in is looking for something specific and will stick to their own plan. The first group is looking for a treasure, which they believe the boggards are likely protecting (see empty box on other side of mud pit in caverns). That's what they're going for. They'll challenge anyone who gets in their way, ie. the boggards. The best way to do it is run a separate encounter where it's just the boggards, including Hears-the-Father, and the first group. One of the boggards will get Zikomo to get the villains to come rescue them from the "invaders". If the villains choose not to, that's their choice. The boggards will likely be killed off, the resources will be somewhat depleted, and the villains will be forced to fight whoever's left.
But yes, it's random. They show up at some point during that week, someone's on patrol or making magic items, and they deal with it or don't deal with it. If they survive and complete Book 2, great. If they lose, you go to Book 3 with new characters.