I've a hard time gathering information in the game about the Dragon Cult and the Dragon War in-game and so far UESP isn't offering much information as well. So what do we actually al know?

When did the war took place?
What was the role of Alduin before the war?
Did people like Ysgramor and Ysmir worship the dragons? and were they part of the Dragon Cult?
What's the relation with Atmora/ the ancient nords and the dragons?
What about the rest of Tamriel and the Dragon-dominion and war?