Some massaging is helpful, yes. Have you checked out the "Enhancing" threads for HotDQ and Rise of Tiamat? They have tons of suggestions on how to handle tricky bits:There's a lot of this adventure that doesn't make sense.
http://www.enworld.org/forum/showth...en-quot-(Practical-stuff-to-try-at-your-table!)
http://www.enworld.org/forum/showth...at-quot-(Practical-stuff-to-try-at-your-table!)
You could point out that the castle flies all day and all night, and the wyverns can't do that; they are going to get tired at some point. Finding a safe place to land in the castle would be the logical course of action. And then maybe letting your players catch a glimpse of the Red Wizard at a tower window would tempt them to go and have a chat with him?"Follow the treasure" is the mission. That's what they are doing. But since no actual speed is given for the castle, only "The wyverns can catch up because the castle is slow", it makes sense to "follow the treasure" not suddenly turn around and blow up the castle.
They might not, and that's okay. Do note that the adventure also includes scenarios where the PCs capture the castle instead, or where Blagothkus crashes it either as himself or as a spirit. The book includes information on combat because that's the most likely way PCs will handle the situation, but the ending of the adventure is really quite open. (For example, my PCs befriended Blagothkus and then took over the castle fairly bloodlessly by dumping some of the hallucinogenic powder from Castle Naerytar into the cultists' food.)Why would anyone except a crew of murder hobos turn around and decide to kill everyone.
But if you want to nudge your group, you could also have one of their contacts, such as Leosin or Ontharr, contact them via a sending spell and advise them to try to sabotage the treasure shipment as soon as they know its final destination, so as to prevent the cult from getting the treasure as a resource and delay their plans.