I wish they'd kept it a mystery too. It was strange that they chose to reveal it in the same episode that Arthur was trying to find out more about his mother. I didn't feel it was rushed as much as I thought it was thrown in with little care. It was like the Morgana/Morgause connection wasn't as important as what it would lead to or something...

Pity, but as you rightly say they can't change history now.

I agree it would be much better if they worked existing characters into the plots rather than inventing them and pulling names out of hats. They have to bear in mind that not everyone at home is in the fandom - ordinary people have to remember these names!