Sophie has written a fic here about the whole Ygraine, Vivienne, Gorlosis, Uther, Agravaine situatuon. She explains that Moraguse is actually Ygraine's daughter with Gorlois (before she married Uther) and that caused her to be barren.

I won't say much more but it just explains Agravaine's motivations and why he didn't like his sister Ygraine and its a great read.

As to when Moraguse was smuggled out its hard to say. It was before Morgana was born and Arthur so magic was allowed, but maybe there were issues with it. All we know is that Uther used magic to conceive Arthur (so he must not have hated it that much)and then why Ygraine died he turned against it.

I agree I hope Mordred returns soon. Maybe he does this series on the next one.