I had a slight problem with the ending as well. Spoiler:
During the scene in the Great Library, it had been explained to the party that the "very death-throes of a Catastrophic Dragon could cause great devastation in the mountains between the monastery and Hammerfast".
Which explains the necessity of killing Actherimos with the Arrow of Time. Even though the party wanted to give the Arrow to Salazar, they realized they'd still be stuck dealing with the dragon, and likely dying when the cavern collapsed.
So with a heavy heart, the heroes did betray their oath to the ghost, and slew the Dragon with the Arrow. I give them a short denouement about how they found themselves back at the beginning of the adventure outside Hammerfast, ready to leave for Inverness. Everyone was alive again, and when they got to the ruins, the Ghost Tower did not appear. The End...or is it?
Our store was running at two tables, so we walked into the next room to see how they were doing, just in time to see them kill the Dragon, Salazar nowhere to be seen. "What happened to Vladistone?", a player asked.
The other DM shrugged. "They gave him the Arrow, and he left the fight."
"Doesn't that mean the cave collapses in an earthquake?"
"Does it? It doesn't say that in the encounter at all."
So the module ends with either an ending where nobody remembers the heroes doing anything heroic and...um...well there isn't another ending! You are either alive and will be considered great heroes for saving the Nentir Vale or...you are dead and have brought great devastation upon the land.
I guess that's a whole new trope: Schrodinger's Ending!
"You can always judge a man by the quality of his enemies." -The Doctor, Remembrance of the Daleks