Most of the settings come from "The Decisive Battle at the Xuanwu Gate". Don't pick on historical facts, because "The Decisive Battle at the Xuanwu Gate" is outrageous and is itself a second creation of structural historical facts. This article is reconstructed under that framework and is a third creation. So don't pick on historical facts. I will try my best to explain some settings.
Concept: Erfeng is not a pure little white rabbit