User blog comment:Chichiryuushintei/High School DxD BorN Episode 5 Review/@comment-3965848-20150504112223/@comment-1359322-20150504160723

Those aren't backsettings, those are plot devices.


 * 1) Great War - God and Maou dies, Three Faction stop fighting, Devil Civil War, eventual Peace Treaty
 * 2) Anti-Satan Faction not Sirzechs vs Old Satan Faction = New Devil Society, Evil Pieces introduced
 * 3) Ddraig vs Albion - Creation of Boosted Gear and Divine Dividing.

In short those are meant to help set up the plot. Backsetting would be like 666, everyone knew about the Great War but no one knew what God did during the war.

Also Ishibumi has teased a High School DxD Zero which is on how Sirzechs met Grayfia. As for your "Journey for Vali", Ishibumi can do a different series like Slash/Dog for Vali since if he's writing for Vali, i doubt Ise will be on it.