aramis erak
Legend
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
There are SEVERAL points where player actions can kick them off the railroad.
The biggest is the fight with Cyanwrath. His kobold buddies ready to slay the families doesn't prevent them from charging Cyanwrath... but that will end their favored status with the Governor.
If they haven't peeved off the cult, they could JOIN the cult... at which point, they are way off the reservation. Not that the rest of the module becomes useless, but it can't be used as-is.
If they kill Cyanwrath in Episodes I or II, there are some changes in Episode III.
If they don't agree to the payments for Episodes II or III, they might not be continuing on the rails... but that doesn't mean the rest of the book is useless. A good DM can use the rest as a sourcebook of sorts for what's happening around them.
And there's plenty to do within the rails that isn't obligatory, and has some lasting impact upon the PCs, even if it doesn't have impact upon the setting. Like finding certain goodies, earning the favor of the townfolk, looting certain treasures. Burning down the huts in the camp (and thus pissing off the camp dwellers).
So, there is Agency. You're just not looking for it. It's there, it's not even all that subtle, and you have to accept that any structured module requires you to accept a reduction in overall agency in order to play; if you can't or won't, then there's always creative writing.... Because RPG rules are always a restriction on agency, and modules are lower still.