User blog comment:GWAINE/Merlin - Episode 5.10 - Press Release/@comment-5465618-20121117033455

If you remember Morgana had a dream that she would be confronted by Emyrs "Dragoon" at the end-- when he will ask her is this what you wanted Morgana. Usually, when Morgana has these visions they come true like Arthur marrying Gwen. So, I would not build your hopes up thinking Morgana is about to find out about Merlin in that episode. It would be too easy...

If Mordred's purpose is to kill Arthur -- he could just do it. He doesn't have to team up with Morgana to do it. He's know about Merlin being Emyrs from the first series -- he will mostly likely not ever admit to knowing this bit of information. I think he knows how crazy Morgana is -- he doesn't want to face her wrath.