There is no plot point that hinges on the PCs doing one particular action. There are no designated Win or Lose conditions within the adventure. What happens next is entirely up to the characters, and the variations are near infinite.
No plot point hangs on it because it is nearly impossible to lose. Everything in this adventure is scripted for the PCs to win. even in the most unlucky case and one of them dies, they still achieve the objective.
This, together with the heavy railroad removes any and all player agency from the game. Consequences for attacking the dragon? There are none as the encounter is scripted. Consequences for not attacking it? None. Consequences for being smart and not enter a town under attack by much too powerful enemies? Not allowed because of railroad.
Same with the half dragon. No matter if you engage or not, win or not, it hardly has any influence on the campaugn thanks to the scripted fight and clone HD