Aren't all story events excuses for drama?
I sorry,It's because the autor try to be coherent, i just want to say all events in this are not logical in the doujin himself or the touhou universe. The way tragedy come is like free violence, i am sorry, i too much limited with my english i can't really express what i want to say. T///T
I think it's more important to view this in terms of this author's other works, rather than the Touhou games themselves. In those, the Tengu are actually less bestial and more organized, there's one in particular that underlines using Momiji that while they have bestial traits they cannot in fact be beasts.
This author generally uses dark stories to convey a message or lesson, so looking at it in terms of "the Tengu wouldn't act this way" or that this is something the dragon god would warn them about misses the point. In this story, what is important is Reimu's dilemma and how people relate to the Gods through times of difficulty.