In Oblivion, when infiltrating Mankar Camerons paradise, he mentions how Lorkhan was a Daedra whos realm was know as "Dawns Beauty (misnamed Tamriel), and how the Divines were actually his minions who betrayed him.

Is there any real lore backing this up? or was Mankar sprouting nonsense

There is (conincidentally?) already a thread about this half way down the page from yours. To answer you shortly, his statements are for you to interpret, although they do run against much of what the lore of the series delivers elsewhere. Though, at the same time, his worldview is predicated on many of those lasting foundations of lore. Lorkhan. The Towers. Dragonborn. CHIM. What do you think?

Well.. Lorkhan isn't an ancestor, I don't think. His heart does factor into creation though, and his avatars have sustained the world.

He's not really a Daedra... and his "Daedric Realm" consists of the remains of his betrayers (or helpers). So I think the "orthodox" (lol) view of creation is more valid than Camoran's.